Optical element, optical pickup device and optical information recording and reproducing apparatus

ABSTRACT

An optical element for use in an optical pickup device to conduct reproducing and/or recording information for a first disk including a protective substrate by the use of a first light flux emitted from a first light source and to conduct reproducing and/or recording information for a second disk including a protective substrate by the use of a second light flux emitted from a first light source, the optical element includes: an optical surface on which a first phase structure is formed to have a function to correct a spherical aberration; an optical surface on which a second phase structure is formed such that when the wavelength of the first light flux changes, the second phase structure generates a spherical aberration in a direction reverse to the direction of a spherical aberration generated by the first phase structure.

FIELD-OF THE INVENTION

The present invention relates an optical element, an optical pickupdevice and an optical information recording and reproducing apparatus.

There has so far been known an optical pickup device capable ofconducting recording/reproducing for plural types of optical disks eachbeing different from others in terms of recording density. An example ofthe optical pickup device is one which conducts recording/reproducing byusing an optical pickup device for DVD (Digital Versatile Disk) and CD(Compact Disc). In recent years, there has been a demand for an opticalpickup device that is compatible for a high density optical disk(hereinafter, optical disks using a violet laser light source as a laserlight source are named generically as “high density disk”) using aviolet laser light source (for example, a violet semiconductor laser ora violet SHG laser), conventional DVD and further for CD, as opticaldisks each being different from others in terms of recording density.

With respect to an optical pickup device having compatibility for thehigh density optical disk and DVD, there are known technologies toutilize, as described in the following Patent Documents 1-3, adiffractive optical element on which a diffractive structure composed ofring-shaped zones in a prescribed number whose centers are on theoptical axis is formed.

(Patent Document 1) TOKKAI No. 2001-60336

(Patent Document 2) TOKKAI No. 2002-298422

(Patent Document 3) TOKKAI No. 2001-93179

Technologies described in the Patent Documents above are represented bya technology to utilize a beam having the diffraction order number lowerthan that of the beam for recording/reproducing for the high densityoptical disk, like utilizing the second order (or third order)diffracted light as a beam for recording/reproducing for high densityoptical disk, and utilizing first order (or second order) diffractedlight as a beam for recording/reproducing for DVD. These technologiesmake it possible to control fluctuations of focus positions formicroscopic wavelength changes in a violet wavelength area, whilesecuring high diffraction efficiency for the beam forrecording/reproducing for each optical disk.

For securing the high diffraction efficiency for the beam forrecording/reproducing for respective high density optical disks, it isnecessary to select a combination of diffraction order number n₁ of thebeam for recording/reproducing for high density optical disk anddiffraction order number n₂ of the beam for recording/reproducing forDVD, so that ratio δφ_(D) of an optical path length added to λ1 by thediffractive structure defined by δφ_(D)={n₁×λ₁/(N₁−1)}/{n₂×λ₂/(N₂−1)} toan optical path length to be added to λ₂ may be a value close to 1, whenλ₁ represents a wavelength of the beam for recording/reproducing forhigh density optical disk, λ₂ represents a wavelength of the beam forrecording/reproducing for DVD and N₁ and N₂ represent respectivelyrefractive indexes of diffractive optical elements respectively for λ₁and λ₂.

Nevertheless, when using the diffractive structure that generatesdiffracted light with combination of diffraction order number n₁ anddiffraction order number n₂ both satisfying the aforesaid relationship,for realizing compatibility between a high density optical disk and DVD,an amount of change of spherical aberration per unit wavelength of thediffractive structure grows greater, thus, the laser light source needsto be selected, resulting in the problem that an increase inmanufacturing cost for the laser light source and an increase inmanufacturing cost for the optical pickup device are caused.

Since this amount of change of spherical aberration per unit wavelengthchange increases in proportion to the fourth power of the numericalaperture (NA) of an objective optical system, the problem stated abovebecomes more obvious, especially in the objective optical system withnumerical aperture (NA) 0.85 represented by the Blu-ray Discrepresenting a standard of a high density optical disk and in a highdensity optical disk with standard using a 0.1 mm-thick protective layer(hereinafter referred to as 0.1 mm standard).

SUMMARY OF THE INVENTION

In view of the problems stated above, an object of the invention is tomake it possible to control an amount of changes of spherical aberrationper unit wavelength change, and thereby to provide an optical element,an optical pickup device and an optical information recording andreproducing apparatus wherein the yield in mass production of laserlight sources is improved and manufacturing cost can be reduced.

To solve the problems stated above, an optical element described in Item1-1 for use in an optical pickup device to conduct reproducing and/orrecording information for a first disk including a protective substratehaving a thickness t₁ by the use of a first light flux having awavelength λ₁ (nm) emitted from a first light source and to conductreproducing and/or recording-information for a second disk including aprotective substrate having a thickness t₂ (t₂≦t₁) by the use of asecond light flux having a wavelength λ2 (λ₂>1) (nm) emitted from afirst light source, the optical element has an optical surface on whicha first phase structure is formed to have a function to correct aspherical aberration caused by a difference in thickness of theprotective substrate between the first optical disk and the secondoptical disk or a function to correct a spherical aberration caused by adifference in wavelength between the first light flux and the secondlight flux; an optical surface on which a second phase structure isformed such that when the wavelength of the first light flux changes,the second phase structure generates a spherical aberration in adirection reverse to the direction of a spherical aberration generatedby the first phase structure.

In the embodiment described in Item 1-2, the first phase structuredescribed in the Item 1-1 is a diffractive structure.

An embodiment described in Item 1-3 is represented by the opticalelement described in Item 1-1, wherein the first phase structure is adiffractive structure that generates n₁ ^(th) order diffracted ray as adiffracted ray having the maximum diffraction efficiency when the firstlight flux comes in and generates n₂ ^(th) order diffracted ray(|n₁|≧|n₂|) as a diffracted-ray having the maximum diffractionefficiency when the second light flux comes in.

An embodiment described in Item 1-4 is represented by the opticalelement described in Item 1-1 or Item 1-2, wherein the second phasestructure is an optical path difference providing structure including aplurality of ring-shaped zones divided with stepped sections each formedin an optical axis direction.

An embodiment described in Item 1-5 is represented by the opticalelement described in Item 1-1, wherein when the wavelength of the firstlight flux changes within a range of (λ₁−5) (nm) to (λ₁+5) (nm), thesecond phase structure has a function to generate a spherical aberrationin a direction reverse to the direction of a spherical aberrationgenerated by the first phase structure.

The embodiments described in Item 1-1 through Item 1-4 make it possibleto correct spherical aberration caused by actions of the diffractivestructure and by a difference of protective layer thickness between ahigh density disk of 0.1 mm standard and DVD, to correct sphericalaberration caused by wavelength dispersion in an objective opticalsystem between a violet wavelength area and a red wavelength area, andto control fluctuations of focus positions for the microscopic change ofa wavelength in the violet wavelength area to be small. However, withrespect to the diffractive structure, a wavelength dependency ofspherical aberration is great, and therefore, a change of sphericalaberration for the wavelength change of about ±5 nm grows greater. Sincethe amount of change of spherical aberration of this kind grows greaterin proportion to NA⁴, a tolerance for an oscillation wavelength of aviolet laser light source becomes severe in the high density disk of 0.1mm standard that uses an objective optical system with NA of 0.85. Inthe optical element in the invention, therefore, a tolerance for anoscillation wavelength of a violet laser light source is eased byemploying the structure wherein the optical path difference providingstructure controls spherical aberration changes to be small forwavelength changes of about ±5 nm for a light flux of incidence. Due tothis, the yield in mass production of violet laser light sources can beimproved and manufacturing cost for violet laser light sources andoptical pickup device can be reduced.

Incidentally, in the present specification, optical disks employingviolet semiconductor lasers or violet SHG lasers as a light source forrecording/reproducing of information are called “high density opticaldisks” generically, recording/reproducing of information is conducted byobjective optical system with NA 0.85, and recording/reproducing ofinformation is conducted by an objective optical system with NA 0.65 inaddition to the optical disk in the standard where a protective layerthickness is about 0.1 mm, and an optical disk in the standard where aprotective layer thickness is about 0.6 mm is also included. Further, inaddition to the optical disk having on its information recording surfacethe protective layer of this kind, an optical disk having on itsinformation recording surface the protective layer having a thickness ofabout several nm-several tens nm and an optical disk having theaforesaid protective layer or having the protective layer whosethickness is zero are also included. Further, in the presentspecification, the high density disk includes a magneto-optical diskthat uses a violet semiconductor laser and a violet SHG laser as a lightsource for recording/reproducing of information.

In the present specification, optical disks in DVD series such asDVD-ROM, DVD-Video, DVD-Audio, DVD-RAM, DVD-R, DVD-RW, DVD+R and DVD+RWare called “DVD” generically, and optical disks in CD series such asCD-ROM, CD-Audio, CD-Video, CD-R and CD-RW are called “CD” generically.

In the present specification again, “a diffractive structure” means astructure that provides a phase difference to a light flux having atleast one wavelength among those of light sources used in an opticalpickup devices. For example, it includes a diffractive structure thatgenerates diffracted light and an optical path difference providingstructure that has steps to provide an optical path difference, and arefracting interface having steps can also be included provided that therefracting interface has the structure to generate a phase difference.

Further, in the present specification, “the diffractive structuregenerates n^(th) order diffracted ray” is synonymous with that adiffraction efficiency of the n^(th) order diffracted ray amongdiffracted rays in various orders which are generated when a light fluxwith wavelength λ enters in the diffractive structure, is the maximum,and “the diffractive structure that generates n^(th) order diffractedray” is synonymous with “the diffractive structure that is blazed sothat the n^(th) order diffracted ray may generate the maximumdiffraction efficiency when a light flux with wavelength λ enters”.Therefore, “using n^(th) order diffracted ray as a beam forrecording/reproducing of an optical disk” is synonymous with that torecord information on an optical disk and/or to reproduce informationrecorded on an optical disk, by converging n^(th) order diffracted rayhaving the maximum diffraction efficiency among diffracted rays invarious orders which are generated when a light flux with wavelength λenters, in the diffractive structure.

Further, in the present specification, “correction of sphericalaberration” means that the spherical aberration is corrected to besmaller than those generated in the case of no corresponding structuressuch as the first phase structure and the second phase structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of primary portions showing the structure of anoptical pickup device.

FIG. 2 is a diagram showing the structure of an objective opticalsystem.

FIG. 3 is a graph showing the wavefront aberration in the case ofwavelength fluctuations of the objective optical system.

FIG. 4 is a plan view of primary portions showing the structure of anoptical pickup device.

FIG. 5 is a diagram showing the structure of an objective opticalsystem.

Each of FIGS. 6 (a)-6 (c) shows the structure of an aberrationcorrecting element.

FIG. 7 is a graph showing the wavefront aberration in the case ofwavelength fluctuations of the objective optical system.

FIG. 8 is a diagram of an optical path of the objective optical system.

FIG. 9 is a graph showing the wavefront aberration in the case ofwavelength fluctuations under the assumption that an optical pathdifference providing structure is not formed.

FIG. 10 is a graph showing the wavefront aberration in the case ofwavelength fluctuations in the example.

FIG. 11 is a diagram of an optical path of the objective optical system.

FIG. 12 is a graph showing the wavefront aberration in the case ofwavelength fluctuations under the assumption that an optical pathdifference providing structure is not formed.

FIG. 13 is a graph showing the wavefront aberration in the case ofwavelength fluctuations in the example.

FIG. 14 is a diagram of an optical path of the objective optical system.

FIG. 15 is a diagram showing the structure of an objective opticalsystem.

FIG. 16 is a graph showing the wavefront aberration in the case ofwavelength fluctuations under the assumption that an optical pathdifference providing structure is not formed.

FIG. 17 is a graph showing the wavefront aberration in the case ofwavelength fluctuations in the example.

SUMMARY OF THE INVENTION

Preferred embodiments of the invention will be explained below.

An embodiment described in Item 1-6 is represented by the opticalelement described in Item 1-3, wherein when N₁ and N₂ are the refractiveindex of the optical element for the first light flux having awavelength λ₁ and the second light flux having a wavelength λ₂respectively and INT(X) is an integer closest to X, the followingformulas are satisfied:

INT(δφ_(D))−δφ_(D)<0  (1)

δφ_(D) ={n ₁×λ₁/(N ₁−1)}/{n ₂×λ₂/(N ₂−1)}  (2), and

wherein the first phase structure has a spherical aberrationcharacteristic such that when the wavelength of an incident light fluxshifts to a longer wavelength side, a spherical aberration changes to beover corrected.

An embodiment described in Item 1-7 is represented by the opticalelement described in Item 1-6, wherein a manufacturing wavelength λ_(B)for the first phase structure satisfies the following formula:

λ_(B)<λ₁

An embodiment described in Item 1-8 is represented by the opticalelement described in Item 1-6 or Item 1-7, wherein the second phasestructure has a spherical aberration characteristic such that when thewavelength of the first light flux shifts to a longer wavelength sidewithin a range of (λ₁−5) (nm) to (λ₁+5) (nm), a spherical aberrationchanges to be under corrected.

Wavelength-dependency of the spherical aberration that is caused by thediffractive structure is utilized for correcting the sphericalaberration resulting from a difference of protective layer thicknessbetween a high density disk in 0.1 mm standard and DVD, and thespherical aberration resulting from the wavelength dispersion of theobjective optical system existing between a violet wavelength area and ared wavelength area. In this case, when expressions (1) and (2)representing relationships hold between wavelength λ₁ and diffractionorder number n₁ of a beam for recording/reproducing for high densityoptical disk and wavelength λ₂ and diffraction order number n₂ of a beamfor recording/reproducing for DVD, as in the embodiment in Item 1-3, itis preferable to make the diffractive structure to have sphericalaberration characteristics in which the spherical aberration is changedin the direction of excessive correction when a wavelength of a lightflux of incidence is shifted toward the long wavelength side.

In this case, it is preferable that manufacturing wavelength (which isalso called a blaze wavelength) λ_(B) of the diffractive structure is aprescribed wavelength that is shorter than λ1, as in the embodiment inItem 1-7, which makes it possible to maintain high diffractionefficiency at the wavelength region for each optical disk. This issynonymous with an occasion wherein the following expression (3) issatisfied when Δ_(D) (μm) represents the step closest to the opticalaxis among steps in the optical axis direction of the diffractivestructure in the optical axis direction, and N_(B) represents therefractive index of the optical element for the wavelength λ_(B)respectively.

Δ_(D) =n ₁×_(B)×10⁻³/(N _(B)−1)  (3)

It is preferable to offset spherical aberration characteristics of, thediffractive-structure, by giving to the optical path differenceproviding structure the spherical aberration characteristics in whichthe spherical aberration is changed in the direction of insufficientcorrection when a wavelength of a light flux of incidence is shiftedtoward the long wavelength side within a range from λ₁−5 (nm) to λ₁+5(nm), as in the embodiment in Item 1-8. Due to this, a tolerance for anoscillation wavelength of a violet laser light source can be eased.

An embodiment described in Item 1-9 is represented by the opticalelement described in Item 1-3, wherein when N₁ and N₂ are the refractiveindex of the optical element for the first light flux having awavelength λ₁ and the second light flux having a wavelength λ₂respectively and INT (X) is an integer closest to X, the followingformulas are satisfied:

INT(δφ_(D))−δφ_(D)>0  (4)

δφ_(D) ={n ₁×λ₁/(N ₁−1)}/{n ₂×λ₂/(N ₂−1)}  (5), and

wherein the first phase structure has a spherical aberrationcharacteristic such that when the wavelength of an incident light fluxshifts to a longer wavelength side, a spherical aberration changes to beunder corrected.

An embodiment described in Item 1-10 is represented by the opticalelement described in Item 1-6, wherein a manufacturing wavelength λ_(B)for the first phase structure satisfies the following formula:

λ₁<λ_(B)<λ₂

An embodiment described in Item 1-11 is represented by the opticalelement described in Item 1-9 or Item 1-10, wherein the second phasestructure has a spherical aberration characteristic such that when thewavelength of the first light flux shifts to a longer wavelength sidewithin a range of (λ₁−5) (nm) to (λ₁+5) (nm), a spherical aberrationchanges to be over corrected.

Wavelength-dependency of the spherical aberration that is caused by thediffractive structure is utilized for correcting the sphericalaberration resulting from a difference of protective layer thicknessbetween a high density disk in 0.1 mm standard and DVD, and thespherical aberration resulting from the wavelength dispersion of theobjective optical system existing between a violet wavelength area and ared wavelength area. In this case, when expressions (4) and (5)representing relationships hold between wavelength λ₁ and diffractionorder number n₁ of a beam for recording/reproducing for high densityoptical disk and wavelength λ₂ and diffraction order number n₂ of a beamfor recording/reproducing for DVD, as in the embodiment in Item 1-9, itis preferable to make the diffractive structure to have sphericalaberration characteristics in which the spherical aberration is changedin the direction of insufficient correction when a wavelength of a lightflux of incidence is shifted toward the long wavelength side.

In this case, it is preferable that manufacturing wavelength (which isalso called a blaze wavelength) λ_(B) of the diffractive structure is aprescribed wavelength that is in a range from λ₁ to λ₂ as in theembodiment in Item 1-10, which makes it possible to maintain highdiffraction efficiency at the wavelength region for each optical disk.This is synonymous with an occasion wherein the following expression (6)is satisfied when Δ_(D) (μm) represents the step closest to the opticalaxis among-steps in the optical axis direction of the diffractivestructure in the optical axis direction, and N_(B) represents therefractive index of the optical element for the wavelength λ_(B)respectively.

Δ_(D) =n ₁×λ_(B)×10⁻³/(N _(B)−1)  (6)

It is preferable to offset spherical aberration characteristics of thediffractive structure, by giving to the optical path differenceproviding structure the spherical aberration characteristics in whichthe spherical aberration is changed in the direction of excessivecorrection when a wavelength of a light flux of incidence is shiftedtoward the long wavelength side within a range from λ1-5 (nm) to λ1+5(nm), as in the embodiment in Item 1-11. Due to this, a tolerance for anoscillation wavelength of a violet laser light source can be eased.

An embodiment described in Item 1-12 is represented by the opticalelement described in any one of Items 1-1 through 1-11, wherein thefirst phase structure is formed on an aspherical surface formed suchthat as the position of an optical path on the aspherical surface isdistant more from the optical axis, the length of the optical pathbecomes longer.

In the embodiment described in Item 1-13, when an addition amount of anoptical path length by the diffractive structure is defined with thefollowing expression (7) by the use of optical path difference functioncoefficients B₂, B₄, B₆, B₈, B₁₀ . . . and diffraction order number n,in the optical element described in any one of Items 1-2, 1-3, and 1-6though 1-11, the sign for B₂ and sign for B₄ are different each other.

φ_(b) =n×(B ₂ h ² +B ₄ h ⁴ +B ₆ h ⁶ +B ₈ ⁸ +B ₁₀ ¹⁰+ . . . )  (7)

The embodiment described in Item 1-13 represents conditions forcontrolling a loss of an amount of light caused by an error in a form ofthe diffractive structure to be small, by preventing a width of aring-shaped zone of the diffractive structure from becoming too small,and thereby, by making it easy to process a die. It is possible toincrease an amount of change of h per unit amount of change of opticalpath difference function φ_(b), by making a sign of second order opticalpath difference function coefficient and a sign of fourth order opticalpath difference function coefficient B₄ to be different each other. Thiscorresponds to that a width of a ring-shaped zone of the diffractivestructure grows greater, resulting in achievement of easy processing ofa die and of control of a loss in an amount of light. For furtherachievement of these effects, it is preferable to establish sizes of B₂and B4 so that optical path difference function φ_(b) may have aninflection point. It is further possible to secure a working distance ofDVD to be large by establishing signs of B₂ and B₄ to be different eachother.

An embodiment described in Item 1-14 is represented by the opticalelement described in any one of Items 1-1 through 1-13, wherein on theoptical surface on which the first phase structure is formed, the firstphase structure is formed within at least a range of 0% to 70% of themaximum effective diameter of the optical surface and the first phasestructure is not formed within at least a range of 85% to 100% of themaximum effective diameter.

In the embodiment described in Item 1-14, the spherical aberrationcaused by a difference of protective layer thickness between a highdensity disk in 0.1 mm standard and DVD is corrected by the action ofthe diffractive structure only within NA necessary forrecording/reproducing for DVD, and therefore, a spot on the informationrecording surface of DVD is not narrowed down more than necessary, andthereby, an amount of comatic aberration caused by a tilt of DVD doesnot become too great. The second light flux passing through an areaoutside NA of DVD has spherical aberration caused by a difference ofprotective-layer thickness between the high density disk and DVD, tobecome flare components which do not contribute to formation of a spoton the information recording surface of DVD. Since this is equivalent tothat the optical element itself has a function to restrict an aperturefor DVD, an optical pickup device equipped with an optical element ofthe invention does not need to be provided separately with a diaphragmcorresponding to NA of DVD, and its structure can be simple.

An embodiment described in Item 1-15 is represented by the opticalelement described in any one of Items 1-1 though 1-14, wherein on theoptical surface on which the second phase structure is formed, thesecond phase structure is formed within at least a range of 0% to 70% ofthe maximum effective diameter of the optical surface and the secondphase structure is not formed within at least a range of 85% to 100% ofthe maximum effective diameter.

In the case of the structure described in Item 1-15,wavelength-dependency of spherical aberration of the diffractivestructure has only to be offset within NA necessary forrecording/reproducing for DVD, and therefore, it is preferable to forman optical path difference providing structure only within NA of DVD.Further, it is preferable to determine the optical path differenceproviding structure so that a phase of wavefront for the first lightflux passing through an area outside NA of DVD and that for the firstlight flux passing through an area within NA of DVD may be aligned inthe wavelength area in at least a range from λ₁-5 (nm) to λ₁+5 (nm).

Incidentally, “maximum effective diameter” mentioned here means aneffective diameter of the optical element for the first light flux withwavelength λ₁.

An embodiment described in Item 1-16 is represented by the opticalelement described in Item 1-8, wherein the second phase structure doesnot provide an optical path difference for the first light flux having awavelength λ₁ and provides an optical path difference for the firstlight flux having a wavelength (λ₁+5) (nm) and the first light fluxhaving a wavelength (λ₁-5) (nm).

An embodiment described in Item 1-17 is represented by the opticalelement described in Item 1-11, wherein the second phase structure doesnot provide an optical path difference for the first light flux having awavelength λ₁ and provides an optical path difference for the firstlight flux having a wavelength (λ₁+5) (nm) and the first light fluxhaving a wavelength (λ₁−5) (nm).

The optical path difference providing structure is preferably thestructure that does not give an optical path difference substantially toλ₁ representing a design wavelength of the optical element as in theembodiments in Item 1-16 and Item 1-17, but gives an optical pathdifference to wavelength λ₁+5 (nm) and wavelength λ₁-5 (nm), which makesit possible to conduct properly an offset of wavelength-dependency ofspherical aberration caused by the diffractive structure. Specifically,it is preferable to determine the step Δ_(P) (μm) closest to the opticalaxis in the optical path difference providing structure so that thefollowing expression is satisfied;

Δ_(P) =m×λ ₁×10⁻³/(N ₁−1)

wherein, m represents a positive integer and N₁ represents refractiveindex of the optical element for the light flux with wavelength λ₁.

An embodiment described in Item 1-18 is represented by the opticalelement described in any one of Items 1-3, 1-6 through 1-11, 1-16 and1-17, wherein the following formulas are satisfied:

|λ₂−λ₁>50 nm and |n ₁ |>|n ₂|

In the embodiment described in Item 1-18, when a difference between λ₂and λ₁ is greater than 50 nm, it is possible to maintain high efficiencyof diffraction in a wavelength region for each optical disk, by usingthe diffracted light whose diffraction order is lower than that of abeam for recording/reproducing for the high density optical disk, as abeam for recording/reproducing for DVD. Since the wavelength-dependencyof the spherical aberration is great for the diffractive structuredetermined to use diffracted light having different order number as abeam for recording/reproducing for each optical disk, compared with anoccasion to use the diffracted light with the same order number, asstated above, a tolerance for an oscillation wavelength of a violetlaser light source becomes extremely severe. In the optical element ofthe invention, however, a tolerance for an oscillation wavelength of aviolet laser light source can be eased by employing the structurewherein the optical path difference providing structure controlsspherical aberration changes to be small for wavelength changes of about±5 nm for a light flux of incidence.

An embodiment described in Item 1-19 is represented by the opticalelement described in Item 1-18, wherein λ₁ is within a range of 350 nmto 450 nm, λ₂ is within a range of 600 nm to 700 nm, and a combination(n₁, n₂) of n₁ and n₂ satisfies the following formula:

(n₁,n₂)=(2,1),(3,2),(5,3),(8,5) or (10,6)

When λ₁ represents a wavelength within a range of 350 nm-450 nmrepresenting a violet wavelength area and λ₂ represents a wavelengthwithin a range of 600 nm-700 nm representing a red wavelength area as inthe embodiment described in Item 1-19, it is preferable that thespecific combination of diffraction order numbers n₁ and n₂ is any oneof (n₁, n₂)=(2, 1), (3, 2), (5, 3), (8, 5), (10, 6), and a selection ofthis combination makes it possible to maintain high diffractionefficiency in a wavelength region for each optical disk. Further, evenin the case where diffraction order number n₁ becomes greater than 10,there is present a combination of diffraction order numbers which canmaintain high diffraction efficiency at a wavelength region for eachoptical disk. However, if the diffraction order number n, is made to besmaller than 10, diffraction efficiency fluctuations do not become toolarge for the wavelength changes of about ±5 (nm) from λ₁, which ispreferable.

An embodiment described in Item 1-20 is represented by the opticalelement described in Item 1-19, wherein the optical wherein the opticalelement is made of a material whose refractive index for the first lightflux having a wavelength λ₁ is within a range of 1.5 to 1.6 and Abbeconstant for d-line (587.6 nm) is within a range of 50 to 60 and thediffractive structure includes a predetermined number of ring-shapedzones divided with stepped sections each formed in an optical axisdirection, and a stepped section Δ_(D) (μm) closest to the optical axisamong the stepped sections satisfies one of the following formulas:

1.25<Δ_(D)<1.65  (8)

2.05<Δ_(D)<2.55  (9)

3.04<Δ_(D)<4.10  (10)

5.70<Δ_(D)<6.45  (11)

7.00<Δ_(D)<8.00  (12)

It is preferable that the embodiment described in Item 1-20 relates tothe more specific structure of the diffractive structure in theaforesaid combination of the diffraction order numbers, and the stepΔ_(D) (μm) closest to the optical axis in the diffractive structuresatisfies the expressions (8)-(12). The combination (n₁, n₂)=(2, 1)corresponds to expression (8), (n₁, n₂)=(3, 2) corresponds to expression(9), (n₁, n₂)=(5, 3) corresponds to expression (10), (n₁, n₂)=(8, 5)corresponds to expression (11), and (n₁, n₂)=(10, 6) corresponds toexpression (12). Incidentally, for manufacturing an optical elementhaving a diffractive structure like the optical element of the inventionwith a high precision, a plastic lens excellent in transferability ispreferable. In most plastic materials usable in a violet wavelengtharea, the refractive index for the first light flux with wavelength λ₁is in a range of 1.5-1.6 and Abbe's number for d line is in a range of50-60.

An embodiment described in Item 1-21 is represented by the opticalelement described in any one of Items 1-18 through 1-20, wherein thesecond phase structure is an optical path difference providing structureincluding a plurality of ring-shaped zones divided with stepped sectionseach formed in an optical axis direction, and when Δp (μm) is a steppedsection closest to the optical axis among the stepped sections in thesecond phase structure, N₁ and N₂ are the refractive index of theoptical element for the first light flux having a wavelength λ₁ and thesecond light flux having a wavelength λ₂ respectively and INT(X) is aninteger closest to X, the following formula satisfies the followingformulas:

0≦|INT(φ_(1P))−φ_(1P)|≦0.4  (13)

0≦|INT(φ_(2P))−φ_(2P))−|≦0.4  (14)

φ_(1P)=Δ_(P)×(N ₁−1)/(λ₁×10⁻³)  (15)

φ_(2P)=Δ_(P)×(N ₂−1)/(λ₂×10⁻³)  (16)

When determining the step Δ_(P) (μm) closest to the optical axis of theoptical path difference providing structure, it is preferable thatneither design wavelength λ₁ on the high density optical disk side nordesign wavelength λ₂ on the DVD side is given a substantial optical pathdifference. Due to this, characteristics of the optical element for λ₂in the invention can be made excellent. Specifically, it is preferableto determine Δ_(P) (μm) so that the expressions (13)-(16) may besatisfied, as in the embodiment of Item 1-21.

An embodiment described in Item 1-22 is represented by the opticalelement described in Item 1-21, wherein when λ₁ is within a range of 350nm to 450 nm, λ₂ is within a range of 600 nm to 700 nm, the opticalelement is made of a material whose refractive index for the first lightflux having a wavelength λ₁ is within a range of 1.5 to 1.6 and Abbeconstant for d-line (587.6 nm) is within a range of 50 to 60 and p is apositive integer, the following formulas are satisfied:

INT(φ_(1P))=5p  (17)

INT(φ_(2P))=3p  (18)

The embodiment described in Item 1-22 relates to the structure that ismore specific in the case where neither design wavelength λ₁ on the highdensity optical disk side nor design wavelength λ₂ on the DVD side isgiven a substantial optical path difference by the optical pathdifference providing structure, and it is preferable that Δ_(P) (μm)satisfies expressions (17) and (18). Incidentally, for manufacturing anoptical element having an optical path difference providing structurelike the optical element of the invention with a high precision, aplastic lens excellent in transferability is preferable. In most plasticmaterials usable in a violet wavelength area, the refractive index forthe first light flux with wavelength λ₁ is in a range of 1.5-1.6 andAbbe's number for d line is in a range of 50-60.

An embodiment described in Item 1-23 is represented by the opticalelement described in any one of Items 1-3, 1-6 through 1-11, 1-13 and1-16 though 1-22, wherein the first phase structure generates a n₃^(th)-order diffracted ray (|n₂|₂≧|n₃|) when the third light flux comesin.

In the embodiment described in Item 1-23, when each of the differencebetween 2 and λ₁, the difference between 3 and 2 and the differencebetween 3 and λ₁ is greater than 50 nm, it is possible to maintain thehigh diffraction efficiency in the wavelength region for each opticaldisk, by using the diffracted light with the order number lower the beamfor recording/reproducing for the high density disk as the beam forrecording/reproducing for DVD, and by using the diffracted-light withthe order number that is the same as or lower than the beam forrecording/reproducing for DVD as the beam for recording/reproducing forCD. In the diffractive structure determined to use the diffracted lighthaving the different order number as a beam for recording/reproducingfor each optical disk of at lease high density optical disk and DVD asstated above, the wavelength-dependency of the spherical aberration isgreat compared with an occasion to use the diffracted light with thesame order number, and a tolerance for the oscillation wavelength of theviolet laser light source becomes to be extremely severe accordingly.However, in the optical element in the invention, a tolerance foroscillation wavelength of the violet laser light source can be eased,because of the structure where changes in spherical aberration forwavelength changes of about ±5 nm in the light flux of incidence in theviolet wavelength area is controlled to be small by the optical pathdifference structure.

An embodiment described in Item 1-24 is represented by the opticalelement described in Item 1-23, wherein λ₁ is within a range of 350 nmto 450 nm, λ₂ is within a range of 600 nm to 700 nm, 3 is within a rangeof 700 nm to 850 nm, and a combination (n₁, n₂, n₃) of n₁, n₂ and n₃satisfies the following formula:

(n₁,n₂,n₃)=(2,1,1),(8,5,4), or (10,6,5)

When λ₁ is a wavelength within a range of 350 nm-450 nm representing aviolet wavelength area, λ₂ is a wavelength within a range of 600 nm 700nm representing a red wavelength area, and λ₃ is a wavelength within arange of 700 nm-850 nm representing a infrared wavelength area, as inthe embodiment described in Item 1-24, it is preferable that a specificcombination of diffraction order numbers n₁, n₂ and n₃ is any one of(n₁, n₂, n₃)=(2, 1, 1) (8, 5, 4) and (10, 6, 5), and a high efficiencyof diffraction can be maintained in the wavelength region for eachoptical disk, by selecting the aforesaid combination. Further, even whenthe diffraction order number n₁ is greater than 10, a combination ofdiffraction order numbers which make it possible to maintain highefficiency of diffraction still exists at a wavelength region for eachoptical disk. In that case, however, changes in diffraction efficiencybecome too great for the wavelength changes of about ±5 (nm) from λ₁,which is not preferable.

An embodiment described in Item 1-25 is represented by the opticalelement described in Item 1-24, wherein the optical element is made of amaterial whose refractive index for the first light flux having awavelength λ₁ is within a range of 1.5 to 1.6 and Abbe constant ford-line (587.6 nm) is within a range of 50 to 60 and the diffractivestructure includes a predetermined number of ring-shaped zones dividedwith stepped sections each formed in an optical axis direction, and astepped section Δ_(D) (μm) closest to the optical axis among the steppedsections satisfies one of the following formulas:

1.25<Δ_(D)<1.65  (19)

5.70<Δ_(D)<6.45  (20)

7.00<Δ_(D)<8.00  (21)

The embodiment described in Item 1-25 relates to the more specificstructure of the diffractive structure in the combination of thediffraction order numbers and it is preferable that step Δ_(D) (μm)closest to the optical axis in the diffractive structure satisfies thefollowing expressions (19)-(21). With respect to correspondence betweenthe combination of diffraction order numbers and expressions (19)-(21),(n₁, n₂, n₃)=(2, 1, 1) corresponds to expression (19), (n₁, n₂, n₃)=(8,5, 4) corresponds to expression (20) and (n₁, n₂, n₃)=(10, 6, 5)corresponds to expression (21). Incidentally, for manufacturing anoptical element having a diffractive structure like the optical elementof the invention with a high precision, a plastic lens excellent intransferability is preferable. In most plastic materials usable in aviolet wavelength area, the refractive index for the first light fluxwith wavelength λ₁ is in a range of 1.5-1.6 and Abbe's number for d lineis in a range of 50-60.

An embodiment described in Item 1-26 is represented by the opticalelement described in any one of Items 1-23 through 1-25, wherein thesecond phase structure is an optical path difference providing structureincluding a plurality of ring-shaped zones divided with stepped sectionseach formed in an optical axis direction, and when Δ_(P) (μm) is astepped section closest to the optical axis among the stepped sectionsin the second phase structure, N₁, N₂ and N₃ are the refractive index ofthe optical element for the first light flux having a wavelength λ₁, thesecond light flux having a wavelength λ₂ and the third light flux havinga wavelength λ₃ respectively and INT(X) is an integer closest to X, thefollowing formula satisfies the following formulas:

0≦|INT(φ_(1P))−φ_(1P)|≦0.4  (22)

0≦|INT(φ_(2P))−φ_(2P)|≦0.4  (23)

0≦|INT(φ_(3P))−φ_(3P)|≦0.4  (24)

φ_(1P)=Δ_(P)×(N ₁−1)/(λ₁×10⁻³)  (25)

φ_(2P)=Δ_(P)×(N ₂−1)/(λ₂×10⁻³)  (26)

φ_(3P)=Δ_(P)×(N ₃−1)/(λ₃×10⁻³)  (27)

When determining the step Δ_(P) (μm) closest to the optical axis of theoptical path difference providing structure, it is preferable that noneof design wavelength λ₁ on the high density optical disk side, designwavelength λ₂ on the DVD side and design wavelength λ₃ on the CD side isgiven a substantial optical path difference. Due to this,characteristics of the optical element in the invention for the secondand third light fluxes with wavelength λ₂ and wavelength λ₃ can be madeexcellent. Specifically, it is preferable to determine Δ_(P) (μm) sothat the expressions (22)-(27) may be satisfied, as in the embodiment ofItem 1-26.

An embodiment described in Item 1-27 is represented by the opticalelement described in Item 1-26, wherein when λ₁ is within a range of 350nm to 450 nm, λ₂ is within a range of 600 nm to 700 nm, 3 is within arange of 700 nm to 850 nm, the optical element is made of a materialwhose refractive index for the first light flux having a wavelength λ₁is within a range of 1.5 to 1.6 and Abbe constant for d-line (587.6 nm)is within a range of 50 to 60 and p is a positive integer, the followingformulas are satisfied:

INT(φ_(1P))=10p  (28)

INT(φ_(2P))=6p  (29)

INT(φ_(3P))=5p  (30)

The embodiment described in Item 1-27 relates to the structure that ismore specific in the case where none of design wavelength λ₁ on the highdensity optical disk side, design wavelength λ₂ on the DVD side anddesign wavelength λ₃ on the CD side is given a substantial optical pathdifference by the optical path difference providing structure, and it ispreferable that Δ_(P) (μm) satisfies expressions (28) and (30).Incidentally, for manufacturing an optical element having an opticalpath difference providing structure like the optical element of theinvention with a high precision, a plastic lens excellent intransferability is preferable. In most plastic materials usable in aviolet wavelength area, the refractive index for the first light fluxwith wavelength λ₁ is in a range of 1.5-1.6 and Abbe's number for d lineis in a range of 50-60.

An embodiment described in Item 1-28 is represented by the opticalelement described in any one of Items 1-1 through 1-27, wherein theoptical element comprises a first structural element on which the firstphase structure is formed and a second structural element on which thesecond phase structure is formed.

As in Item 1-28, the optical element of the invention may also becomposed of the first constituent element on which the diffractivestructure is formed and the second constituting element on which theoptical path difference providing structure is formed. By forming thediffractive structure and the optical path difference providingstructure to be different constituent elements, optical performances anderrors of forms of each structure can easily grasped in the course ofmanufacturing, which makes it easy to manufacture the optical element.

An embodiment described in Item 1-29 is represented by the opticalelement described in any one of Items 1-1 through 1-28, wherein theoptical element is a plastic lens.

For manufacturing an optical element having a diffractive structure oran optical path difference providing structure with a high precision,like the embodiment in Item 1-29, a plastic lens excellent intransferability is preferable.

An embodiment described in Item 1-30 is represented by the opticalelement described in any one of Items 1-1 through 1-29, wherein theoptical element is a structural element of an objective optical systemfor use in an optical pickup device.

An embodiment described in Item 1-30 makes it possible to provide anoptical pickup device having compatibility for a high density opticaldisk and DVD by using the optical element described in any one of Items1-1-through 1-29 as a constituent element of an objective opticalsystem.

An embodiment described in Item 1-31 is represented by the opticalelement described in Item 1-30, wherein the objective optical systemcomprises an aberration correcting element and a light convergingelement to converge a light flux emitted from the aberration correctingelement on an information recording surface of an optical disk.

When an optical element of the invention is used as a constituent factorof the objective optical system as in the embodiment in Item 1-31, it ispreferable that the optical element is arranged between the lightconverging element that converges a light flux emitted from a laserlight source on an information recording surface of an optical disk andthe laser light source. With respect to the light converging elementhaving NA 0.85, if a diffractive structure or an optical path differenceproviding structure is formed on an optical surface of the lightconverging element, an influence of the eclipse of a ray of light bysteps makes a loss of an amount of light large, because an angle formedby a normal on an aspheric surface and an optical axis grows greater.However, it is possible to control such loss of an amount of light to besmall by making the optical element of the invention to be powerless.

An embodiment described in Item 1-32 is represented by the opticalelement described in Item 1-30, wherein the objective optical system isone group-structured light converging element and the light convergingelement is the optical element.

The embodiment of Item 1-32 makes it possible to obtain an opticalelement having a same effect as that of Item 1-31.

An embodiment described in Item 1-33 is an optical pickup device toconduct reproducing and/or recording information for a first diskincluding a protective substrate having a thickness t₁ by the use of afirst light flux having a wavelength λ₁ (nm) emitted from a first lightsource and to conduct reproducing and/or recording information for asecond disk including a protective substrate having a thickness t₂(t₂≧t₁) by the use of a second light flux having a wavelength λ₂ (λ₂>λ₁)(nm) emitted from a first light source, the optical pickup devicecomprises the optical element described in Item 1-1.

The embodiment described in Item 1-33 makes it possible to obtain anoptical pickup device having the same effect as that in any one of Items1-1 through 1-32.

An embodiment described in Item 1-34 is represented by the opticalpickup device described in Item 1-33, wherein the optical pickup devicefurther conducts reproducing and/or recording information for a seconddisk including a protective substrate having a thickness t₃ (t₃≧t₂) bythe use of a second light flux having a wavelength λ₃ (λ₃>λ₂) (nm)emitted from a first light source, and the third light flux comes in theoptical element on a state of a divergent light flux.

In order to provide compatibility even for the third optical disk (CD),it is preferable to cause the third light flux to enter the opticalelement as a divergent light flux as in the embodiment in Item 1-36. Dueto this, a working distance for CD having a thick protective layer canbe secured sufficiently.

The embodiment described in Item 1-35 includes the optical pickup devicedescribed in Item 1-33 or Item 1-34, so as to conduct at least one ofrecording information for an optical disk and reproducing informationfor an optical disk.

According to the embodiment of Item 1-35, an optical informationrecording reproducing device having a same effect to the embodiment ofItem 1-33 or Item 1-34 can be provided.

An embodiment described in Item 1-36 is an optical element for use in anoptical pickup device to conduct reproducing and/or recordinginformation for a first disk including a protective substrate having athickness t₁ by the use of a first light flux having a wavelength λ₁(nm) emitted from a first light source and to conduct reproducing and/orrecording information for a second disk including a protective substratehaving a thickness t₂ (t₂≧t₁) by the use of a second light flux having awavelength λ₂ (λ₂>λ₁) (nm) emitted from a first light source, theoptical element including an optical surface on which a diffractivestructure is formed to generate n₁ ^(th) order diffracted ray as adiffracted ray having the maximum diffraction efficiency when the firstlight flux comes in and to generate n₂ ^(th) order diffracted ray(|n₁≧|n₂|) as a diffracted ray having the maximum diffraction efficiencywhen the second light flux comes in; and an optical surface on which anoptical path difference providing structure including a plurality ofring-shaped zones divided with stepped sections each formed in anoptical axis direction is formed.

The embodiment described in Item 1-36 makes it possible to obtain anoptical element having the same effect as that of Item 1-1.

An embodiment described in Item 1-37 is represented by the opticalelement in Item 1-36, wherein when the wavelength of the first lightflux changes within a range of (λ₁−5) (nm) to (λ₁+5) (nm), the opticalpath difference providing structure has a function to generate aspherical aberration in a direction reverse to the direction of aspherical aberration generated by the diffractive structure.

The embodiment described in Item 1-37 makes it possible to obtain anoptical element having the same effect as that of Item 1-5.

An embodiment described in Item 1-38 is represented by the opticalelement in Item 1-37, wherein when N₁ and N₂ are the refractive index ofthe optical element for the first light flux having a wavelength λ₁ andthe second light flux having a wavelength λ₂ respectively and INT(X) isan integer closest to X, the following formulas are satisfied:

INT(δφ_(D))−δφ_(D)<0  (1)

δφ_(D) ={n ₁×λ₁/(N ₁-1)}/{n ₂×λ₂/(N ₂−1)}  (2), and

wherein the diffractive structure has a spherical aberrationcharacteristic such that when the wavelength of an incident light fluxshifts to a longer wavelength side, a spherical aberration changes to beover corrected.

The embodiment described in Item 1-38 makes it possible to obtain anoptical element having the same effect as that of Item 1-6.

An embodiment described in Item 1-39 is represented by the opticalelement in Item 1-38, wherein the diffractive structure includes aplurality of ring-shaped zones divided with stepped sections each formedin an optical axis direction and when Δ_(D) (μm) is a stepped sectionclosest to the optical axis among the stepped sections, a manufacturingwavelength is λ_(B) (nm) (λ_(B)<λ₁), N_(B) is the refractive index ofthe optical element for the manufacturing wavelength λ_(B), thefollowing formula is satisfied:

Δ_(D) =n ₁×λ_(B)×10⁻³/(N _(B)−1)  (3)

The embodiment described in Item 1-39 makes it possible to obtain anoptical element having the same effect as that of Item 1-7.

An embodiment described in Item 1-40 is represented by the opticalelement in Item 1-38 or Item 39, wherein the diffractive structure has aspherical aberration characteristic such that when the wavelength of thefirst light flux shifts to a longer wavelength side within a range of(λ₁−5) (nm) to (λ₁+5) (nm), a spherical aberration changes to be undercorrected.

The embodiment described in Item 1-40 makes it possible to obtain anoptical element having the same effect as that of Item 1-8.

An embodiment described in Item 1-41 is represented by the opticalelement in Item 1-37, wherein when N₁ and N₂ are the refractive index ofthe optical element for the first light flux having a wavelength λ₁ andthe second light flux having a wavelength λ₂ respectively and INT(X) isan integer closest to X, the following formulas are satisfied:

INT(δφ_(D))−δφ_(D)>0  (4)

δφ_(D) ={n ₁×λ₁(N ₁−1)}/{n ₂×λ₂/(N ₂−1)}  (5), and

wherein the diffractive structure has a spherical aberrationcharacteristic such that when the wavelength of an incident light fluxshifts to a longer wavelength side, a spherical aberration changes to beunder corrected.

The embodiment described in Item 1-41 makes it possible to obtain anoptical element having the same effect as that of Item 1-9.

An embodiment described in Item 1-42 is represented by the opticalelement in Item 1-41, wherein the diffractive structure includes aplurality of ring-shaped zones divided with stepped sections each formedin an optical axis direction and when Δ_(D) (μm) is a stepped sectionclosest to the optical axis among the stepped sections, a manufacturingwavelength is λ_(B) (nm) (λ₁<λ_(B)<λ₂), N_(B) is the refractive index ofthe optical element for the manufacturing wavelength λ_(B), thefollowing formula is satisfied:

Δ_(D) =n ₁×λ_(B)×10⁻³/(N _(B)−1)  (6)

The embodiment described in Item 1-42 makes it possible to obtain anoptical element having the same effect as that of Item 1-10.

An embodiment described in Item 1-43 is represented by the opticalelement in any one of Item 1-41 or Item 1-42, wherein the optical pathdifference providing structure has a spherical aberration characteristicsuch that when the wavelength of the first light flux shifts to a longerwavelength side within a range of (λ₁−5) (nm) to (λ₁+5) (nm), aspherical aberration changes to be over corrected.

The embodiment described in Item 1-43 makes it possible to obtain anoptical element having the same effect as that of Item 1-11.

An embodiment described in Item 1-44 is represented by the opticalelement in one of Items 1-36 through 1-43, wherein the optical pathdifference providing structure is formed on an aspherical-surface formedsuch that as the position of an optical path on the aspherical surfaceis distant more from the optical axis, the length of the optical pathbecomes longer.

The embodiment described in Item 1-44 makes it possible to obtain anoptical element having the same effect as that of Item 1-12.

An embodiment described in Item 1-45 is represented by the opticalelement in one of Items 1-36 through 1-44, wherein on the opticalsurface on which the optical path difference providing structure isformed, the diffractive structure is formed within at least a range of0% to 70% of the maximum effective diameter of the optical surface andthe diffractive structure is not formed within at least a range of 85%to 100% of the maximum effective diameter.

The embodiment described in Item 1-45 makes it possible to obtain anoptical element having the same effect as that of Item 1-14.

An embodiment described in Item 1-46 is represented by the opticalelement in Item 1-36, wherein the optical element is made of a materialwhose refractive index for the first light flux having a wavelength λ₁is within a range of 1.5 to 1.6 and Abbe constant for d-line (587.6 nm)is within a range of 50 to 60 and the diffractive structure includes apredetermined number of ring-shaped zones divided with stepped sectionseach formed in an optical axis direction, and a stepped section Δ_(D)(μm) closest to the optical axis among the stepped sections satisfiesone of the following formulas:

1.25<Δ_(D)<1.65  (8)

2.05<Δ_(D)<2.55  (9)

3.40<Δ_(D)<4.10  (10)

5.70<Δ_(D)<6.45  (11)

7.00<Δ_(D)<8.00  (12)

The embodiment described in Item 1-46 makes it possible to obtain anoptical element having the same effect as that of Item 1-20.

An embodiment described in Item 1-47 is represented by the opticalelement in one of Items 1-36 through 1-46, wherein when λ₁ is within arange of 350 nm to 450 nm, λ₂ is within a range of 600 nm to 700 nm, theoptical element is made of a material whose refractive index for thefirst light flux having a wavelength λ₁ is within a range of 1.5 to 1.6and Abbe constant for d-line (587.6 nm) is within a range of 50 to 60,Δ_(P) (μm) is a stepped section closest to the optical axis among thestepped sections in the optical path difference providing structure, N₁and N₂ are the refractive index of the optical element for the firstlight flux having a wavelength λ₁ and the second light flux having awavelength λ₂ respectively and INT(X) is an integer closest to X and pis a positive integer, the following formulas are satisfied:

φ_(1P)=Δ_(P)×(N ₁−1)/(λ₁×10⁻³)  (15)

φ_(2P)=Δ_(P)×(N ₂−1)/(λ₂×10⁻³)  (16)

INT(φ_(1P)=5p  (17)

INT(φ_(2P))=3p  (18)

The embodiment described in Item 1-47 makes it possible to obtain anoptical element having the same effect as that of Item 1-22.

An embodiment described in Item 1-48 is represented by the opticalelement in Item 1-36 through 1-47, wherein the diffractive structuregenerates a n₃ ^(th) order diffracted ray (|n₂|≧|n₃|) when the thirdlight flux comes in.

The embodiment described in Item 1-48 makes it possible to obtain anoptical element having the same effect as that of Item 1-23.

An embodiment described in Item 1-49 is represented by the opticalelement in Item 1-48, wherein λ₁ is within a range of 350 nm to 450 nm,λ₂ is within a range of 600 nm to 700 nm, λ₃ is within a range of 700 nmto 850 nm, and a combination (n₁, n₂, n₃) of n₁, n₂ and n₃ satisfies thefollowing formula:

(n₁,n₂,n₃)=(2,1,1),(8,5,4), or (10,6,5)

The embodiment described in Item 1-49 makes it possible to obtain anoptical element having the same effect as that of Item 1-24.

An embodiment described in Item 1-50 is represented by the opticalelement in Item 1-49, wherein the optical element is made of a materialwhose refractive index for the first light flux having a wavelength λ₁is within a range of 1.5 to 1.6 and Abbe constant for d-line (587.6 nm)is within a range of 50 to 60 and the diffractive structure includes apredetermined number of ring-shaped zones divided with stepped sectionseach formed in an optical axis direction, and a stepped section Δ_(D)(μm) closest to the optical axis among the stepped sections satisfiesone of the following formulas:

1.25<Δ_(D)<1.65  (19)

5.70<Δ_(D)<6.45  (20)

7.00<Δ_(D)<8.00  (21)

The embodiment described in Item 1-50 makes it possible to obtain anoptical element having the same effect as that of Item 1-25.

An embodiment described in Item 1-51 is represented by the opticalelement in any one of Items 1-48 through 1-50, wherein when Δ_(P) (μm)is a stepped section closest to the optical axis among the steppedsections in the optical path difference providing structure, N₁, N₂ andN₃ are the refractive index of the optical element for the first lightflux having a wavelength λ₁, the second light flux having a wavelengthλ₂ and the third light flux having a wavelength λ3 respectively andINT(X) is an integer closest to X, the following formula satisfies thefollowing formulas:

0<|INT(φ_(1P))−φ_(1P)|≦0.4  (22)

0<|INT(φ_(2P))−φ_(2P)|≦0.4  (23)<

0<|INT(φ_(3P))−φ_(3P)|≦0.4  (24)

φ_(1P)=Δ_(P)×(N ₁−1)/(λ₁×10⁻³)  (25)

φ_(2P)=Δ_(P)×(N ₂−1)/(λ₂×10⁻³)  (26)

φ_(3P)=Δ_(P)×(N ₃−1)/(λ₃×10⁻³)  (27)

The embodiment described in. Item 1-51 makes it possible to obtain anoptical element having the same effect as that of Item 1-26.

An embodiment described in Item 1-52 is represented by the opticalelement in Item 1-51, wherein when λ₁ is within a range of 350 nm to 450nm, λ₂ is within a range of 600 nm to 700 nm, λ₃ is within a range of700 nm to 850 nm, the optical element is made of a material whoserefractive index for the first light flux having a wavelength λ₁ iswithin a range of 1.5 to 1.6 and Abbe constant for d-line (587.6 nm) iswithin a range of 50 to 60 and p is a positive integer, the followingformulas are satisfied:

INT(φ_(1P))=10p  (28)

INT(φ_(2P))=6p  (29)

INT(φ_(3P))=5p  (30)

The embodiment described in Item 1-52 makes it possible to obtain anoptical element having the same effect as that of Item 1-27.

An embodiment described in Item 1-53 is represented by the opticalelement in any one of Items 1-36 through 1-52, wherein the opticalelement is a plastic lens.

The embodiment described in Item 1-53 makes it possible to obtain anoptical element having the same effect as that of Item 1-29.

An embodiment described in Item 1-54 is an optical pickup device toconduct reproducing and/or recording information for a first diskincluding a protective substrate having a thickness t₁ by the use of afirst light flux having a wavelength λ₁ (nm) emitted from a first lightsource and to conduct reproducing and/or recording information for asecond disk including a protective substrate having a thickness t₂(t₂≧t₁) by the use of a second light flux having a wavelength λ₂ (λ₂>λ₁)(nm) emitted from a first light source, the optical pickup devicecomprises the optical element described in claim 36 and the third lightflux comes in the optical element on a state of a divergent light flux.

The embodiment described in Item 1-54 makes it possible to obtain anoptical pickup device having the same effect as that of Item 1-33.

Then, another preferred embodiments of the invention will be explainedbelow.

To solve the problems stated above, the embodiment described in Item 2-1has at least one optical surface on which a diffractive structure thatgenerates n₁ ^(th) order diffracted light when the first light flux withwavelength λ₁ (nm) enters and generates n₂ ^(th) order diffracted light(|n₁|≧|n₂|) when the second light flux with wavelength λ₂ (nm) (λ₂>λ₁)enters is formed, and at least one optical surface on which an opticalpath difference providing structure that is composed of pluralring-shaped zones formed by division by steps in the optical axisdirection is formed.

In the embodiment described in Item 2-2, the optical path differenceproviding structure in the optical element described in the Item 2-1 hasa function to control a change to be small for spherical aberration thatis generated at the diffractive structure when the wavelength of thefirst light flux is changed within a range from λ₁−5 (nm) to λ₁+5 (nm).

The embodiments described in Item 2-1 through Item 2-2 make it possibleto correct spherical aberration caused by actions of the diffractivestructure and by a difference of protective layer thickness between ahigh density disk of 0.1 mm standard and DVD, to correct sphericalaberration caused by wavelength dispersion in an objective opticalsystem between a violet wavelength area and a red wavelength area, andto control fluctuations of focus positions for the microscopic change ofa wavelength in the violet wavelength area to be small. However, withrespect to the diffractive structure, a wavelength dependency ofspherical aberration is great, and therefore, a change of sphericalaberration for the wavelength change of about ±5 nm grows greater. Sincethe amount of change of spherical aberration of this kind grows greaterin proportion to NA⁴, a tolerance for an oscillation wavelength of aviolet laser light source becomes severe in the high density disk of 0.1mm standard that uses an objective optical system with NA of 0.85. Inthe optical element in the invention, therefore, a tolerance for anoscillation wavelength of a violet laser light source is eased byemploying the structure wherein the optical path difference providingstructure controls spherical aberration changes to be small forwavelength changes of about ±5 nm for a light flux of incidence. Due tothis, the yield in mass production of violet laser light sources can beimproved and manufacturing cost for violet laser light sources andoptical pickup device can be reduced.

An embodiment described in Item 2-3 is represented by the opticalelement described in Item 2-1 or Item 2-2, wherein the followingexpressions are satisfied when N₁ and N₂ represent refractive indexes ofthe optical element respectively for the first light flux withwavelength λ₁ and the second light flux with wavelength λ₂, and when INT(X) is made to be an integer closest to X,

INT(δφ_(D))−δφ_(D)<0  (1)

δφ_(D) ={n ₁×λ₁/(N ₁−1)}/{n ₂×λ₂/(N ₂−1)}  (2)

and the diffractive structure has spherical aberration characteristicsin which the spherical aberration is changed in the direction ofexcessive correction when a wavelength of a light flux of incidence isshifted toward the long wavelength side.

An embodiment described in Item 2-4 is represented by the opticalelement described in Item 2-3, wherein the following expression (3) issatisfied substantially when the diffractive structure is composed of aplurality of ring-shaped zones divided by steps in the optical axialdirection, and when Δ_(D) (μm) represents the step closest to theoptical axis among the aforesaid steps, λ_(B) (nm) represents aprescribed wavelength that is shorter than the aforementionedwavelength. λ₁ and N_(B) represents the refractive index of the opticalelement for the wavelength λ_(B) respectively.

Δ_(D) =n ₁×λ_(B)×10⁻³/(N _(B)−1)  (3)

An embodiment described in Item 2-5 is represented by the opticalelement described in Item 2-3 or Item 2-4, wherein the optical pathdifference providing structure has spherical aberration characteristicsin which the spherical aberration is changed in the direction ofinsufficient correction when a wavelength of a light flux of incidenceis shifted toward the long wavelength side within a range from λ₁−5 (nm)to λ₁+5 (nm).

Wavelength-dependency of the spherical aberration that is caused by thediffractive structure is utilized for correcting the sphericalaberration resulting from a difference of protective layer thicknessbetween a high density disk in 0.1 mm standard and DVD, and thespherical aberration resulting from the wavelength dispersion of theobjective optical system existing between a violet wavelength area and ared wavelength area. In this case, when expressions (1) and (2)representing relationships hold between wavelength λ₁ and diffractionorder number n₁ of a beam for recording/reproducing for high densityoptical disk and wavelength λ₂ and diffraction order number n₂ of a beamfor recording/reproducing for DVD, as in the embodiment in Item 2-3, itis preferable to make the diffractive structure to have sphericalaberration characteristics in which the spherical aberration is changedin the direction of excessive correction when a wavelength of a lightflux of incidence is shifted toward the long wavelength side.

In this case, it is preferable that manufacturing wavelength (which isalso called a blaze wavelength) λ_(B) of the diffractive structure is aprescribed wavelength that is shorter than λ₁, as in the embodiment inItem 2-4, which makes it possible to maintain-high diffractionefficiency at the wavelength region for each optical disk. Specificallyit is preferable that the step closest to the optical axis among stepsin the optical axis direction of the diffractive structure in theoptical axis direction Δ_(D) (μm) satisfies the expression (3).

It is preferable to offset spherical aberration characteristics of thediffractive structure, by giving to the optical path differenceproviding structure the spherical aberration characteristics in whichthe spherical aberration is changed in the direction of insufficientcorrection when a wavelength of a light flux of incidence is shiftedtoward the long wavelength side within a range from λ1−5 (nm) to λ1+5(nm), as in the embodiment in Item 2-5. Due to this, a tolerance for anoscillation wavelength of a violet laser light source can be eased.

An embodiment described in Item 2-6 is represented by the opticalelement described in Item 2-1 or Item 2-2, wherein the followingexpressions are satisfied when refractive indexes of the optical elementrespectively for the first light flux with wavelength λ₁ and the secondlight flux with wavelength λ₂ are represented respectively by N₁ and N₂and when INT(X) is made to be an integer closest to X,

INT(δφ_(D))−δφ_(D)>0  (4)

δφ_(D) ={n ₁×λ₁(N ₁−1)}/{n ₂×λ₂/(N ₂−1)}  (5)

and the diffractive structure has spherical aberration characteristicsin which the spherical aberration is changed in the direction ofinsufficient correction when a wavelength of a light flux of incidenceis shifted toward the long wavelength side.

An embodiment described in Item 2-7 is represented by the opticalelement described in Item 2-6, wherein the following expression (6) issatisfied substantially when the diffractive structure is composed of aplurality of ring-shaped zones divided by steps in the optical axialdirection, and when Δ_(D) (μm) represents the step closest to theoptical axis among the aforesaid steps, λ_(B) (nm) represents aprescribed wavelength within a range from λ₁ to λ₂ and N_(B) representsthe refractive index of the optical element for the wavelength λ_(B)respectively.

Δ_(D) =n ₁×λ_(B)×10⁻³(N _(B)−1)  (6)

In the embodiment described in Item 2-8, the optical path differenceproviding structure has spherical aberration characteristics in whichthe spherical aberration is changed in the direction of excessivecorrection when a wavelength of a light flux of incidence is shiftedtoward the long wavelength side within a range from λ₁−5 (nm) to λ₁+5(nm), in the optical element described in Item 2-6 or Item 2-7.

Wavelength-dependency of the spherical aberration that is caused by thediffractive structure is utilized for correcting the sphericalaberration resulting from a difference of protective layer thicknessbetween a high density disk in 0.1 mm standard and DVD, and thespherical aberration resulting from the wavelength dispersion of theobjective optical system existing between a violet wavelength area and ared wavelength area. In this case, when expressions (4) and (5)representing relationships hold between wavelength λ₁ and diffractionorder number n₁ of a beam for recording/reproducing for high densityoptical disk and wavelength λ₂ and diffraction order number n₂ of a beamfor recording/reproducing for DVD, as in the embodiment in Item 2-6, itis preferable to make the diffractive structure to have sphericalaberration characteristics in which the spherical aberration is changedin the direction of insufficient correction when a wavelength of a lightflux of incidence is shifted toward the long wavelength side.

In this case, it is preferable that manufacturing wavelength (which isalso called a blaze wavelength) λ_(B) of the diffractive structure is aprescribed wavelength that is in a range from λ₁ to λ₂ as in theembodiment in Item 2-7, which makes it possible to maintain highdiffraction efficiency at the wavelength region for each optical disk.This is synonymous with an occasion wherein the following expression (6)is satisfied when Δ_(D) (μm) represents the step closest to the opticalaxis among steps in the optical axis direction of the diffractivestructure in the optical axis direction, and N_(B) represents therefractive index of the optical element for the wavelength λ_(B)respectively.

Δ_(D) =n ₁×λ_(B)×10⁻³/(N _(B)−1)  (6)

It is preferable to offset spherical aberration characteristics of thediffractive structure, by giving to the optical path differenceproviding structure the spherical aberration characteristics in whichthe spherical aberration is changed in the direction of excessivecorrection when a wavelength of a light flux of incidence is shiftedtoward the long wavelength side within a range from λ₁−5 (nm) to λ₁+5(nm), as in the embodiment in Item 2-5. Due to this, a tolerance for anoscillation wavelength of a violet laser light source can be eased.

An embodiment described in Item 2-9 is represented by the opticalelement described in any one of Items 2-1 through 2-8, wherein theoptical element stated above has an aspheric surface within a range ofheight of at least 0%-75% of the maximum effective diameter.

In the embodiment described in Item 2-10, the diffractive structurestated above is formed on the aspheric surface in the optical elementdescribed in Item 2-9.

As in the embodiment described in Item 2-9, it is preferable that anoptical element on which a diffractive structure is formed has anaspheric surface whose optical path length is longer as it is fartheraway from the optical axis, within a range of height of at least 0%-75%of the maximum effective diameter. Owing to this, the sphericalaberration resulting from a difference of protective layer thicknessbetween a high density optical disk in 0.1 mm standard and DVD, and thespherical aberration resulting from the wavelength dispersion of theobjective optical system existing between a violet wavelength area and ared wavelength area can be corrected properly, by combining actions ofthe aspheric surface and actions of the diffractive structure.

In addition, it is most preferable that a diffractive structure isformed on such aspheric surface as in the embodiment described in Item2-10, for exhibiting the maximum correcting effect for the sphericalaberration.

Incidentally, “the maximum effective diameter” mentioned here means aneffective diameter of an optical element for the first light source withwavelength λ₁.

In the embodiment described in Item 2-11, when an addition amount of anoptical path length by the diffractive structure is defined with thefollowing expression (7) by the use of optical path difference functioncoefficients B₂, B₄, B₆, B₈, B₁₀ . . . and diffraction order number n,in the optical element described in any one of Items 2-1 through 2-10,the sign for B₂ and sign for B₄ are different each other.

φ_(b) =n×(B ₂ h ² +B ₄ h ⁴ +B ₆ h ⁶ +B ₈ h ⁸ +B ₁₀ ¹⁰+ . . . )  (7)

The embodiment described in Item 2-11 represents conditions forcontrolling a loss of an amount of light caused by an error in a form ofthe diffractive structure to be small, by preventing a width of aring-shaped zone of the diffractive structure from becoming too small,and thereby, by making it easy to process a die. It is possible toincrease an amount of change of h per unit amount of change of opticalpath difference function φ_(b), by making a sign of second order opticalpath difference function coefficient and a sign of fourth order opticalpath difference function coefficient B₄ to be different each other. Thiscorresponds to that a width of a ring-shaped zone of the diffractivestructure grows greater, resulting in achievement of easy processing ofa die and of control of a loss in an amount of light. For furtherachievement of these effects, it is preferable to establish sizes of B₂and B₄ so that optical path difference function φ_(b) may have aninflection point. It is further possible to secure a working distance ofDVD to be large by establishing signs of B₂ and B₄ to be different eachother.

An embodiment described in Item 2-12 is represented by the opticalelement described in any one of Items 2-1 through 2-11, wherein on theoptical surface on which the diffractive structure is formed, thediffractive structure is formed within a range of height of at least0%-75% of the maximum effective diameter, and the diffractive structureis not formed in a range from a height of at least 85% of the maximumeffective diameter to a height of 100% of the maximum effectivediameter.

In the embodiment described in Item 2-12, the spherical aberrationcaused by a difference of protective layer thickness between a highdensity disk in 0.1 mm standard and DVD is corrected by the action ofthe diffractive structure only within NA necessary forrecording/reproducing for DVD, and therefore, a spot on the informationrecording surface of DVD is not narrowed down more than necessary, andthereby, an amount of comatic aberration caused by a tilt of DVD doesnot become too great. The second light flux passing through an areaoutside NA of DVD has spherical aberration caused by a difference ofprotective layer thickness between the high density disk and DVD, tobecome flare components which do not contribute to formation of a spoton the information recording surface of DVD. Since this is equivalent tothat the optical element itself has a function to restrict an aperturefor DVD, an optical pickup device equipped with an optical element ofthe invention does not need to be provided separately with a diaphragmcorresponding to NA of DVD, and its structure can be simple.

An embodiment described in Item 2-13 is represented by the opticalelement described in Item 2-12, wherein the optical path differenceproviding structure is formed within a range of height of at least0%-75% of the maximum effective diameter, and the optical pathdifference providing structure is not formed in a range from a height ofat least 85% of the maximum effective diameter to a height of 100% ofthe maximum effective diameter.

In the case of the structure described in Item 2-12,wavelength-dependency of spherical aberration of the diffractivestructure has only to be offset within NA necessary forrecording/reproducing for DVD, and therefore, it is preferable to forman optical path difference providing structure only within NA of DVD asthe embodiment of Item 2-13. Further, it is preferable to determine theoptical path difference providing structure so that a phase of wavefrontfor the first light flux passing through an area outside NA of DVD andthat for the first light flux passing through an area within NA of DVDmay be aligned in the wavelength area in at least a range from λ₁−5 (nm)to λ₁+5 (nm).

Incidentally, “maximum effective diameter” mentioned here means aneffective diameter of the optical element for the first light flux withwavelength λ₁.

An embodiment described in Item 2-14 is represented by the opticalelement described in Item 2-5 or Item 2-8, wherein the optical pathdifference providing structure does not give an optical path differenceto the first light flux with wavelength λ₁, and it adds an optical pathdifference to the first light fluxes respectively with wavelength λ₁+5(nm) and wavelength λ₁−5 (nm).

The optical path difference providing structure is preferably thestructure that does not give an optical path difference substantially toλ₁ representing a design wavelength of the optical element as in theembodiment in Item 2-14, but gives an optical path difference towavelength λ₁+5 (nm) and wavelength λ₁−5 (nm), which makes it possibleto conduct properly an offset of wavelength-dependency of sphericalaberration caused by the diffractive structure. Specifically, it ispreferable to determine the step Δ_(P) (μm) closest to the optical axisin the optical path difference providing structure so that the followingexpression is satisfied;

Δ_(P) =m×λ ₁×10⁻³/(N ₁−1)

wherein, m represents a positive integer and N₁ represents refractiveindex of the optical element for the light flux with wavelength λ.

An embodiment described in Item 2-15 is represented by the opticalelement described in any one of Items 2-1 through 2-14 wherein thering-shaped zones of the optical path difference providing structurechange aperiodically in terms their widths. This “aperiodic change”means, in this case, that the width is not expressed by a function ofheight h from the optical axis.

An embodiment described in Item 2-16 is represented by the opticalelement described in any one of Items 2-1-2-15, wherein the steps of theoptical path difference providing structure change places in terms ofdirections within an effective diameter.

An embodiment described in Item 2-17 is represented by the opticalelement described in any one of Items 2-1 through 2-15, wherein thesteps of the optical path difference providing structure are entirelythe same in terms of directions within an effective diameter.

In design of the optical path difference providing structure, a width ofeach ring-shaped zone and the direction of the step depend on awavefront form which is to be corrected by the optical path differenceproviding structure. A specific structure of the optical path differencestructure may also be one wherein a width of each ring-shaped zonechanges aperiodically as in the embodiment of Item 2-15. Further, thespecific structure may be either one wherein the direction of the stepchanges places within the effective diameter as in the embodiment ofItem 2-16 or one wherein the steps are entirely the same in terms ofdirections within an effective diameter as in the embodiment of Item2-17.

Incidentally, “effective diameter” mentioned here means the effectivediameter of the optical element for the first light flux with wavelengthλ₁.

An embodiment described in Item 2-18 is represented by the opticalelement described in any one of Items 2-1 through 2-17 wherein |λ2−λ1 isgreater than 50 nm and |n₁|>|n₂| is satisfied.

In the embodiment described in Item 2-18, when a difference between λ₂and λ₁ is greater than 50 nm, it is possible to maintain high efficiencyof diffraction in a wavelength region for each optical disk, by usingthe diffracted light whose diffraction order is lower than that of abeam for recording/reproducing for the high density optical disk, as abeam for recording/reproducing for DVD. Since the wavelength-dependencyof the spherical aberration is great for the diffractive structuredetermined to use diffracted light having different order number as abeam for recording/reproducing for each optical disk, compared with anoccasion to use the diffracted light with the same order number, asstated above, a tolerance for an oscillation wavelength of a violetlaser light source becomes extremely severe. In the optical element ofthe invention, however, a tolerance for an oscillation wavelength of aviolet laser light source can be eased by employing the structurewherein the optical path difference providing structure controlsspherical aberration changes to be small for wavelength changes of about±5 nm for a light flux of incidence.

An embodiment described in Item 2-19 is represented by the opticalelement described in Item 2-18, wherein the λ₁ is in a range of 350nm-450 nm, the λ₂ is in a range of 600 nm-700 nm, and the combination ofthe n₁ and n₂ is any one of (n₁, n₂)=(2, 1), (3, 2), (5, 3), (8, 5),(10, 6).

When λ₁ represents a wavelength within a range of 350 nm-450 nmrepresenting a violet wavelength area and λ₂ represents a wavelengthwithin a range of 600 nm-700 nm representing a red wavelength area as inthe embodiment described in Item 2-19, it is preferable that thespecific combination of diffraction order numbers n₁ and n₂ is any oneof (n₁, n₂)=(2, 1), (3, 2), (5, 3), (8, 5), (10, 6), and a selection ofthis combination makes it possible to maintain high diffractionefficiency in a wavelength region for each optical disk. Further, evenin the case where diffraction order number n, becomes greater than 10,there is present a combination of diffraction order numbers which canmaintain high diffraction efficiency at a wavelength region for eachoptical disk. However, if the diffraction order number n, is made to besmaller than 10, diffraction efficiency fluctuations do not become toolarge for the wavelength changes of about ±5 (nm) from λ₁, which ispreferable.

An embodiment described in Item 2-20 is represented by the opticalelement described in Item 2-19, wherein the optical element is formed bya material wherein the refractive index for the first light flux withwavelength λ₁ is within a range of 1.5-1.6, and Abbe's number for d line(587.6 nm) is in a range of 50-60, the diffractive structure is composedof ring-shaped zones in a prescribed quantity divided by steps in theoptical axis direction, and step Δ_(D) (μm) closest to the optical axisamong the aforesaid steps satisfies any one of the following expressions(8)-(12).

1.25<Δ_(D)<1.65  (8)

2.05<Δ_(D)<2.55  (9)

3.40<Δ_(D)<4.10  (10).

5.70<Δ_(D)<6.45  (11)

7.00<Δ_(D)<8.00  (12)

It is preferable that the embodiment described in Item 2-20 relates tothe more specific structure of the diffractive structure in theaforesaid combination of the diffraction order numbers, and the stepΔ_(D) (μm) closest to the optical axis in the diffractive structuresatisfies the expressions (8) (12) The combination (n₁, n₂)=(2, 1)corresponds to expression (8), (n₁, n₂)=(3, 2) corresponds to expression(9), (n₁, n₂)=(5, 3) corresponds to expression (10), (n₁, n₂)=(8, 5)corresponds to expression (11), and (n₁, n₂)=(10, 6) corresponds toexpression (12). Incidentally, for manufacturing an optical elementhaving a diffractive structure like the optical element of the inventionwith a high precision, a plastic lens excellent in transferability ispreferable. In most plastic materials usable in a violet wavelengtharea, the refractive index for the first light flux with wavelength λ₁is in a range of 1.5-1.6 and Abbe's number for d line is in a range of50-60.

An embodiment described in Item 2-21 is represented by the opticalelement described in any one of Items 2-18 through 2-20, wherein thefollowing expressions (13)-(16) are satisfied when Δ_(P) (μm) representsthe step closest to the optical axis among the aforesaid steps of theoptical path difference providing structure, N₁ and N₂ representrespectively refractive indexes of the optical element respectively forthe first light flux with wavelength λ₁ and the second light flux withwavelength λ₂ and INT(X) represents an integer closest to X.

0≦|INT(φ_(1P))−φ_(1P)|≦0.4  (13)

0≦|INT(φ_(2P))−φ_(2P)|≦0.4  (14)

φ_(1P)=Δ_(P)×(N ₁−1)/(λ₁×10⁻³)  (15)

φ_(2P)Δ_(P)=(N ₂−1)/(λ₂×10⁻³)  (16)

When determining the step Δ_(P) (μm) closest to the optical axis of theoptical path difference providing structure, it is preferable thatneither design wavelength λ₁ on the high density optical disk side nordesign wavelength λ₂ on the DVD side is given a substantial optical pathdifference. Due to this, characteristics of the optical element for λ₂in the invention can be made excellent.

Specifically, it is preferable to determine Δ_(P) (μm) so that theexpressions (13)-(16) may be satisfied, as in the embodiment of Item2-21.

An embodiment described in Item 2-22 is represented by the opticalelement described in Item 2-21, wherein the following expressions (17)and (18) are satisfied when λ₁ is within a range of 350 nm-450 nm, λ₂ iswithin a range of 600 nm-700 nm, the optical element is made ofa-material in which the refractive index for the first light flux withwavelength λ₁ is within a range of 1.5-1.6, and Abbe's number for d line(587.6 nm) is in a range of 50-60, and p represents a positive integer.

INT(φ_(1P))=5p  (17)

INT(φ_(2P))=3p  (18)

The embodiment described in Item 2-22 relates to the structure that ismore specific in the case where neither design wavelength λ₁ on the highdensity optical disk side nor design wavelength λ₂ on the DVD side is .. . given a substantial optical path difference by the optical pathdifference providing structure, and it is preferable that Δ_(P) (μm)satisfies expressions (17) and (18). Incidentally, for manufacturing anoptical element having an optical path difference providing structurelike the optical element of the invention with a high precision, aplastic lens excellent in transferability is preferable. In most plasticmaterials usable in a violet wavelength area, the refractive index forthe first light flux with wavelength λ₁ is in a range of 1.5-1.6 andAbbe's number for d line is in a range of 50-60.

An embodiment described in Item 2-23 is represented by the opticalelement described in any one of Items 2-1 through 2-22, wherein thediffractive structure generates n₃ ^(th) order (|n₂|≧|n₃|) diffractedlight, when the third light flux with wavelength λ₃ (nm) (λ₃>λ₂) enters.

An embodiment described in Item 2-24 is represented by the opticalelement described in Item 2-23, wherein each of |λ₂−λ₁|, |λ₃−λ₂| and|λ₃−λ₁| is greater than 50 nm and satisfies |n₁|>|n₂|≧|n₃|.

In the embodiments described in Items 2-23 and 2-24, when each of thedifference between λ₂ and λ₁, the difference between 3 and 2 and thedifference between λ₃ and λ₁ is greater than 50 nm, it is possible tomaintain the high diffraction efficiency in the wavelength region foreach optical disk, by using the diffracted light with the order numberlower the beam for recording/reproducing for the high density disk asthe beam for recording/reproducing for DVD, and by using the diffractedlight with the order number that is the same as or lower than the beamfor recording/reproducing for DVD as the beam for recording/reproducingfor CD. In the diffractive structure determined to use the diffractedlight having the different order number as a beam forrecording/reproducing for each optical disk of at lease high densityoptical disk and DVD as stated above, the wavelength-dependency of thespherical aberration is great compared with an occasion to use thediffracted light with the same order number, and a tolerance for theoscillation wavelength of the violet laser light source becomes to beextremely severe accordingly. However, in the optical element in theinvention, a tolerance for oscillation wavelength of the violet laserlight source can be eased, because of the structure where changes inspherical aberration for wavelength changes of about ±5 nm in the lightflux of incidence in the violet wavelength area is controlled to besmall by the optical path difference structure.

An embodiment described in Item 2-25 is represented by the opticalelement described in Item 2-24, wherein λ₁ is within a range of 350nm-450 nm, λ₂ is within a range of 600 nm-700 nm and λ₃ is within arange of 700 nm-850 nm and a combination of n₁, n₂ and n₃ is any one of(n₁, n₂, n₃)=(2, 1, 1) (8, 5, 4) and (10, 6, 5).

When λ₁ is a wavelength within a range of 350 nm-450 nm representing aviolet wavelength area, λ₂ is a wavelength within a range of 600 nm-700nm representing a red wavelength area, and λ₃ is a wavelength within arange of 700 nm-850 nm representing a infrared wavelength area, as inthe embodiment described in Item 2-25, it is preferable that a specificcombination of diffraction order numbers n₁, n₂ and n₃ is any one of(n₁, n₂, n₃)=(2, 1, 1) (8, 5, 4) and (10, 6, 5), and a high efficiencyof diffraction can be maintained in the wavelength region for eachoptical disk, by selecting the aforesaid combination. Further, even whenthe diffraction order number n₁ is greater than 10, a combination ofdiffraction order numbers which make it possible to maintain highefficiency of diffraction still exists at a wavelength region for eachoptical disk. In that case, however, changes in diffraction efficiencybecome too great for the wavelength changes of about ±5 (nm) from λ₁,which is not preferable.

An embodiment described in Item 2-26 is represented by the opticalelement described in Item 2-25, wherein the optical element is made of amaterial whose refractive index for the first light source withwavelength λ₁ is in a range of 1.5-1.6, and whose Abbe's number for dline (587.6 nm) is in a range of 50-60, the diffractive structure iscomposed of ring-shaped zones in a prescribed quantity divided by thesteps in the optical axis direction, and step Δ_(D) (μm) closest to theoptical axis among the aforesaid steps satisfies any one of thefollowing expressions (19)-(21).

1.25<Δ_(D)<1.65  (19)

5.70<Δ_(D)<6.45  (20)

7.00<Δ_(D)<8.00  (21)

The embodiment described in Item 2-26 relates to the more specificstructure of the diffractive structure in the combination of thediffraction order numbers and it is preferable that step Δ_(D) (μm)closest to the optical axis in the diffractive structure satisfies thefollowing expressions (19)-(21). With respect to correspondence betweenthe combination of diffraction order numbers and expressions (19)-(21),(n₁, n₂, n₃)=(2, 1, 1) corresponds to expression (19), (n₁, n₂, n₃)=(8,5, 4) corresponds to expression (20) and (n₁, n₂, n₃)=(10, 6, 5)corresponds to expression (21). Incidentally, for manufacturing anoptical element having a diffractive structure like the optical elementof the invention with a high precision, a plastic lens excellent intransferability is preferable. In most plastic materials usable in aviolet wavelength area, the refractive index for the first light fluxwith wavelength λ₁ is in a range of 1.5-1.6 and Abbe's number for d lineis in a range of 50-60.

An embodiment described in Item 2-27 is represented by the opticalelement described in any one of Items 2-23 through 2-26, wherein thefollowing expressions (22)-(27) are satisfied when Δ_(P) (μm) representsthe step closest to the optical axis among the aforesaid steps of theoptical path difference providing structure, N₁, N₂ and N₃ representrespectively refractive indexes of the optical element respectively forthe first light flux with wavelength λ₁, the second light flux withwavelength λ₂ and the third light flux with wavelength λ₃ and INT (X)represents an integer closest to X.

0≦|INT(φ_(1P))−φ_(1P)|≦0.4  (22)

0≦|INT(φ_(2P))−φ_(2P)|≦0.4  (23)

0≦|INT(φ_(3P))−φ_(3P)|≦0.4  (24)

φ_(1P)=Δ_(P)(N ₁−1)/(λ₁×10⁻³)  (25)

φ_(2P)=Δ_(P)(N ₂−1)/(λ₂×10⁻³)  (26)

φ_(3P)=Δ_(P)(N ₃−1)/(λ₃×10⁻³)  (27)

When determining the step Δ_(P) (μm) closest to the optical axis of theoptical path difference providing structure, it is preferable that noneof design wavelength λ₁ on the high density optical disk side, designwavelength λ₂ on the DVD side and design wavelength λ₃ on the CD side isgiven a substantial optical path difference. Due to this,characteristics of the optical element in the invention for the secondand third light fluxes with wavelength λ₂ and wavelength λ₃ can be madeexcellent. Specifically, it is preferable to determine Δ_(P) (μm) sothat the expressions (22)-(27) may be satisfied, as in the embodiment ofItem 2-27.

An embodiment described in Item 2-28 is represented by the opticalelement described in Item 2-27, wherein the following expressions (28)and (30) are satisfied when λ₁ is within a range of 350 nm-450 nm, λ₂ iswithin a range of 600 nm-700 nm, 3 is within a range of 700 nm-850 nm,the optical element is made of a material in which the refractive indexfor the first light flux with wavelength λ₁ is within a range of1.5-1.6, and Abbe's number for d line (587.6 nm) is in a range of 50-60,and p represents a positive integer.

INT(φ_(1P))=10p  (28)

INT(φ_(2P))=6p  (29)

INT(φ_(3P))=5p  (30)

The embodiment described in Item 2-28 relates to the structure that ismore specific in the case where none of design wavelength λ₁ on the highdensity optical disk side, design wavelength λ₂ on the DVD side anddesign wavelength λ₃ on the CD side is given a substantial optical pathdifference by the optical path difference providing structure, and it ispreferable that Δ_(P) (μm) satisfies expressions (28) and (30).Incidentally, for manufacturing an optical element having an opticalpath difference providing structure like the optical element of theinvention with a high precision, a plastic lens excellent intransferability is preferable. In most plastic materials usable in aviolet wavelength area, the refractive index for the first light fluxwith wavelength λ1 is in a range of 1.5-1.6 and Abbe's number for d lineis in a range of 50-60.

An embodiment described in Item 2-29 is represented by the opticalelement described in any one of Items 2-1 through 2-28, wherein thereare provided at least two constituent elements of the first constitutingelement on which the diffractive structure is formed and the secondconstituting element on which the optical path difference providingstructure is formed.

As in Item 2-29, the optical element of the invention may also becomposed of the first constituent element on which the diffractivestructure is formed and the second constituting element on which theoptical path difference providing structure is formed. By forming thediffractive structure and the optical path difference providingstructure to be different constituent elements, optical performances anderrors of forms of each structure can easily grasped in the course ofmanufacturing, which makes it easy to manufacture the optical element.

An embodiment described in Item 2-30 is represented by the opticalelement described in any one of Items 2-1 through 2-29, wherein theoptical element is a plastic lens.

For manufacturing an optical element having a diffractive structure oran optical path difference providing structure with a high precision,like the embodiment in Item 2-30, a plastic lens excellent intransferability is preferable.

An embodiment described in Item 2-31 is represented by the opticalelement described in any one of Items 2-1 through 2-30, wherein there isshown a constituent element of the objective optical element used in anoptical pickup device that conducts reproducing and/or recording ofinformation for the first optical disk having t₁-thick protective layerby the use of the first light flux with wavelength λ₁ (nm) emitted fromthe first light source and conducts reproducing and/or recording ofinformation for the second optical disk having t₂-thick (t₂≧t₁)protective layer by the use of the second light flux with wavelength λ₂(nm) (λ₂>λ₁) emitted from the second light source.

An embodiment described in Item 2-31 makes it possible to provide anoptical pickup device having compatibility for a high density opticaldisk and DVD by using the optical element described in any one of Items2-1 through 2-30 as a constituent element of an objective opticalsystem.

An embodiment described in Item 2-32 is represented by the opticalelement described in Item 2-31, wherein the objective optical system iscomposed of an aberration correcting element and of a light convergingelement that converges a light flux emerging from the aberrationcorrecting element on an information recording surface of an opticaldisk, and this aberration correcting element is the aforesaid opticalelement.

An embodiment described in Item 2-33 is represented by the opticalelement described in Item 2-32, wherein each of the aberrationcorrecting element and the light-converging element has an opticalfunctional portion and a flange portion formed on a periphery of theoptical functional portion, and the flange portion of the aberrationcorrecting element and that of the light-converging element are formedto be capable of fixing the aberration correcting element and thelight-converging element at the prescribed relative positions.

An embodiment described in Item 2-34 is represented by the opticalelement described in Item 2-32, wherein the aberration correctingelement and the light converging element are formed to be fixed byholding members at the prescribed relative positions.

When an optical element of the invention is used as a constituent factorof the objective optical system as in the embodiment in Item 2-32, it ispreferable that the optical element is arranged between the lightconverging element that converges a light flux emitted from a laserlight source on an information recording surface of an optical disk andthe laser light source. With respect to the light converging elementhaving NA 0.85, if a diffractive structure or an optical path differenceproviding structure is formed on an optical surface of the lightconverging element, an influence of the eclipse of a ray of light bysteps makes a loss of an amount of light large, because an angle formedby a normal on an aspheric surface and an optical axis grows greater.However, it is possible to control such loss of an amount of light to besmall by making the optical element of the invention to be powerless.

In that case, it is preferable that the optical element and the lightconverging element are united solidly through the direct contact oftheir flanges or through separated holding members, as in the embodimentin Item 2-33 or Item 2-34. Due to this, even when the light-convergingelement is driven for tracking, no shifting of optical axis is notcaused between the light converging element and the optical element,thus, excellent tracking characteristics can be obtained. Further, it ispreferable that d_(s) is established so that ratio d_(s)/Σd may besmaller than 1.5 when d_(s) represents a distance between the opticalelement and the light-converging element on the optical axis and Σdrepresents a distance between an optical surface of the optical elementon the laser light source side and an optical surface of thelight-converging element on the optical disk side on the optical axis,and owing to this, weight of flange portions and holding members can bereduced, and a load for an actuator to drive the objective opticalsystem can be lightened accordingly.

An embodiment described in Item 2-35 is an optical pickup device thatconducts reproducing and/or recording of information for the firstoptical disk having t₁-thick protective layer by using the first lightflux with wavelength λ₁ (nm) emitted from the first light source, andconducts reproducing and/or recording of information for the secondoptical disk having t₂-thick (t₂>t₁) protective layer by using thesecond light flux with wavelength λ₂ (nm) (λ₂>λ₁) emitted from thesecond light source, wherein the optical element described in any one ofItems 2-1 through 2-34 is provided.

The embodiment described in Item 2-35 makes it possible to obtain anoptical pickup device having the same effect as that in any one of Items2-1 through 2-34.

An embodiment described in Item 2-36 is represented by the opticalpickup device described in Item 2-35, wherein there is conductedreproducing and/or recording of information for the third optical diskhaving t₃-thick (t₃≧t₂) protective layer by using the third light fluxwith wavelength λ₃ (nm) (λ₃>λ₂) emitted from the third light source, andthe optical pickup device is structured so that the third light flux mayenter the optical element under the state of a divergent light flux.

In order to provide compatibility even for the third optical disk (CD),it is preferable to cause the third light flux to enter the opticalelement as a divergent light flux as in the embodiment in Item 2-36. Dueto this, a working distance for CD having a thick protective layer canbe secured sufficiently.

Carrying the optical pickup device described in Item 2-35 or Item 2-36,the embodiment described in Item 2-37 can conduct at least one ofrecording of information for an optical disk and reproducing ofinformation recorded on an optical disk.

The embodiment described in Item 2-37 makes it possible to obtain anoptical information recording/reproducing apparatus having the sameeffect as in Item 2-35 or Item 2-36.

The invention makes it possible to obtain an optical element, an opticalpickup device and optical information recording/reproducing apparatuswherein an yield in mass production of laser light sources can beimproved, and manufacturing cost can be reduced.

Preferred embodiments to practice the invention will be explained indetail as follows, referring to the drawings.

First Embodiment

FIG. 1 is a diagram showing schematically the structure of the firstoptical pickup device PU1 that can conduct recording/reproducing ofinformation properly for high density optical disk HD (first opticaldisk), DVD (second optical disk) and CD (third optical disk). Theoptical specifications of the high density optical disk HD includewavelength λ₁=405 nm, protective layer PL1 thickness t₁=0.1 mm andnumerical aperture NA₁=0.85, the optical specifications of DVD includewavelength λ₂=655 nm, protective layer PL2 thickness t₂=0.6 mm andnumerical aperture NA₂=0.65 and the optical specifications of CD includewavelength λ₃=785 nm, protective layer PL3 thickness t₃=1.2 mm andnumerical aperture NA₃=0.45. However, the invention is not limited tothe aforesaid combination of the wavelength, the thickness of aprotective layer and the numerical aperture.

As shown in FIG. 1, the optical pickup device PU1 is composed of moduleMD1 for high density optical disk use in which violet semiconductorlaser LD1 (first light source) that emits the first light flux andphotodetector PD1 are united solidly, module MD2 for DVD use in whichred semiconductor laser LD2 (second light source) that emits the secondlight flux and photodetector PD2 are united solidly, and module MD3 forCD use in which infrared semiconductor laser LD3 (third light source)that emits the third light flux and photodetector PD3 are unitedsolidly, beam forming element BSH that forms a sectional shape of thelaser light flux emitted from the violet semiconductor laser LD1 from anoval shape to a circular shape, collimating optical system COL, uniaxialactuator UAC, objective optical system OBJ1, biaxial actuator AC, firstbeam combiner BC1, second beam combiner BC2 and of diaphragm STO.

When conducting recording/reproducing of information for high densityoptical disk HD in the optical pickup device PU1, violet semiconductorlaser LD1 is driven first to emit light as its light path is drawn withsolid lines in FIG. 1. A divergent light flux emitted from the violetsemiconductor laser LD1 passes through the beam forming element BSH tobe formed therein from an oval shape to a circular shape in terms of itssectional form, then, it passes through the first beam combiner BC1 andis transmitted through the collimating optical system COL to beconverted into a collimated light flux. After that, it passes throughthe second beam combiner BC2 in succession, and is formed on informationrecording surface RL1 through protective layer PL1 of the high densityoptical disk HD to become a spot.

Incidentally, detailed explanation of objective optical system OBJ1 willbe given later.

The objective optical system OBJ1 conduct focusing and tracking throughoperations of biaxial actuator AC. The reflected light flux modulated oninformation recording surface RL1 by information pits passes againthrough the objective optical system OBJ1, the second beam combiner BC2,collimating optical system COL, the first beam combiner BC1 and beamforming element BSH in succession to be converged on light-receivingsurface of photodetector PD1. Thus, information recorded on high densityoptical disk by the use of output signals of the photodetector PD1 canbe read.

When conducting recording/reproducing of information for DVD, redsemiconductor laser LD2 is driven first to emit light as its light pathis drawn with dotted lines in FIG. 1. A divergent light flux emittedfrom the red semiconductor laser LD2 is reflected on the first beamcombiner BC1, then, is transmitted through the collimating opticalsystem COL to be converted into a collimated light flux, and it passesthrough the second beam combiner BC2 to be converged by objectiveoptical system OBJ1 on information recording surface RL2 to become aspot through protective layer PL2 of DVD.

Then, the objective optical system OBJ1 conduct focusing and trackingthrough operations of biaxial actuator AC arranged on the peripherythereof. The reflected light flux modulated on information recordingsurface RL2 by information pits passes again through the objectiveoptical system OBJ1, the second beam combiner BC2 and collimatingoptical system COL, and then, is branched by the first beam combiner BC1to be converged on a light-receiving surface of photoreceptor PD2. Thus,information recorded on DVD can be read by the use of output signals ofphotodetector PD2.

When conducting recording/reproducing of information for CD, module MD3for CD is operated so that infrared semiconductor laser LD3 may bedriven to emit light as its light path is drawn with two-dot chain linesin FIG. 1. A divergent light flux emitted from the infraredsemiconductor laser LD3 is reflected on the second beam combiner BC2 tobe converged, then, is transmitted through the collimating opticalsystem COL to be converted into a collimated light flux, and it passesthrough the second beam combiner BC2 to be converged by objectiveoptical system OBJ1 on information recording surface RL3 to become aspot through protective layer PL3 of CD. Then, the objective opticalsystem OBJ1 conducts focusing and tracking through operations of biaxialactuator AC arranged on the periphery thereof. The reflected light fluxmodulated on information recording surface RL3 by information pitspasses is transmitted again through the objective optical system OBJ1,and then, is reflected by the second beam combiner BC2, to beconverged-on the light-receiving surface of photodetector PD3 of moduleMD3 for CD. Thus, information recorded on CD by the use of outputsignals of the photodetector PD3 can be read.

The structure of the objective optical system OBJ1 will be explainednext, as follows.

The objective optical system OBJ1 whose side view is shown in FIG. 2 isone that is composed of an optical element of the invention. Theobjective optical system OBJ1 is composed of aberration correctingelement L1 representing a plastic lens and of light-converging elementL2 representing an aspheric glass lens with NA 0.85 having a function toconverge a laser light flux transmitted through the aberrationcorrecting element L1 on an information recording surface of an opticaldisk.

On optical surface S1 of the aberration correcting element L1 on thelaser light source side, there is formed diffractive structure DOE thatis composed of plural ring-shaped zones (see the enlarged diagram inFIG. 2).

Further, on optical surface S2 of the aberration correcting element L1on the optical disk side, there is formed optical path differenceproviding structure PST that is composed of plural ring-shaped zones.Incidentally, the aberration correcting element L1 and thelight-converging element L2 are united solidly through holding member B.

The diffractive structure DOE is a structure to correct sphericalaberration resulting from a difference of protective layer thicknessesbetween a high density optical disk meeting 0.1 mm standard requirementsand DVD, and the diffractive structure DOE is formed on an asphericsurface for the purpose of correcting the spherical aberration properly.By using the second order diffracted light, the first order diffractedlight and the first order-diffracted light, respectively as a beam forrecording/reproducing for the high density optical disk, a beam forrecording/reproducing for DVD and a beam for recording/reproducing forCD, high diffraction efficiency is secured in the wavelength region foreach optical disk.

Incidentally, in optical pickup device PU1, there is employed astructure wherein the third light flux enters the objective opticalsystem OBJ1 under the condition of a divergent light flux, forcorrecting spherical aberration resulting from a difference ofprotective layer thicknesses between high density optical disk HD andCD. This makes it possible to secure a large working distance in thecase of conducting recording/reproducing of information for CD.

Since the diffractive structure DOE satisfies expressions (1) and (2),when a wavelength of the light flux entering the diffractive structureDOE is shifted to the longer wavelength side by Δλ from the designwavelength λ1 on the high density optical disk side, as shown withtwo-dot chain lines A1 in FIG. 3, the diffractive structure DOE hasspherical aberration characteristics that the spherical aberrationchanges toward the excessive correction.

Within a range of height up to 70% of the maximum effective diameter(corresponding to NA₁) each ring-shaped zone of optical path differenceproviding structure PST is changed in a way that an optical path lengthfor the ring-shaped zone that is farther from the optical axis isshorter than that for the ring-shaped zone that is closer to the opticalaxis, while, in the area outside the aforesaid range, each ring-shapedzone is changed in a way that an optical path length for the ring-shapedzone that is farther from the optical axis is longer than that for thering-shaped zone that is closer to the optical axis, thus, the opticalpath difference providing structure PST has spherical aberrationcharacteristics that the spherical aberration changes toward theinsufficient correction.

As stated above, with respect to the diffractive structure DOE thatgenerates beams each having a different diffraction order for twowavelengths having a large wavelength difference, an amount of change ofspherical aberration per a unit wavelength change is large. However, itis possible to cancel the wavelength-dependency of the sphericalaberration as shown with solid lines A3 in FIG. 3, by causing theoptical path difference providing structure PST to have sphericalaberration characteristics opposite to the diffractive structure DOE sothat wavelength-dependency of spherical aberration of the diffractivestructure DOE may be cancelled.

Further, the diffractive structure DOE is blazed with prescribedwavelength λ_(B) that is shorter than λ₁, and step Δ_(D) (μm) closest tothe optical axis of the diffractive structure DOE satisfies expression(3).

Further, step Δ_(P) (μm) closest to the optical axis of the optical pathdifference providing structure PST satisfies expressions (28)-(30), andis established to the depth that does not give an optical pathdifference substantially, to design wavelength λ₁ on the high densityoptical disk side, design wavelength λ₂ on the DVD side and designwavelength λ₃ on the CD side.

Incidentally, optical pickup device PU1 is provided with dichroic filterDFL that restricts an aperture in the case of conductingrecording/reproducing for DVD and CD, and this dichroic filter DFL isunited with objective optical system OBJ1 through holding member B to bedriven by biaxial actuator AC in the direction perpendicular to theoptical axis.

Further, it is also possible to conduct aperture restriction in the caseof conducting recording/reproducing for DVD, by giving a function toflare the second light flux to the diffractive structure DOE formed onan area outside numerical aperture NA₂ of DVD and to optical pathdifference providing structure PST, and it is also possible to conductaperture restriction in the case of conducting recording/reproducing forCD, by giving a function to flare the third light flux to thediffractive structure DOE formed on an area outside numerical apertureNA₃ of CD and to optical path difference providing structure PST. Or, itmay be possible to form, on an optical surface of objective opticalsystem OBJ1, a diffractive structure having the function to restrict anaperture and an optical path difference providing structure PST,separately form the diffractive structure DOE and the optical pathdifference providing structure PST.

Further, collimating optical system COL is structured to be capable ofbeing moved by uniaxial actuator UAC in the optical axis direction. Thismakes it possible to maintain excellent recording/reproducingcharacteristics constantly for high density optical disk HD, because itbecomes possible to correct spherical aberration of a spot formed oninformation recording surface RL1 of high density optical disk.

Causes for occurrence of spherical aberration to be corrected bypositional adjustment of collimating optical system COL include, forexample, wavelength fluctuations caused by errors in manufacturing ofviolet semiconductor laser LD1, refractive index changes and refractiveindex distribution of objective optical system OBJ1 resulting fromtemperature changes, focus jump between layers in recording/reproducingfor multi-layer disk such as two-layer disk and four-layer disk, andthickness fluctuations by manufacturing errors of protective layers PL1,and thickness distribution.

It is also possible to correct spherical aberration of a spot formed oninformation recording surface RL2 of DVD by positional adjustment ofcollimating optical system COL, which can improve recording/reproducingcharacteristics for DVD.

Second Embodiment

Next, Second Embodiment of the invention will be explained, in which thesame components as those in the First Embodiment will be given the samesymbols, and explanations thereof will be omitted.

A second optical pickup device PU2 shown in FIG. 4 is an optical pickupdevice capable of conducting recording/reproducing of informationproperly for high density optical disk HD (first optical disk) and DVD(second optical disk). The second optical pickup device PU2 is composedof violet semiconductor laser LD1 (first light source) that emits thefirst light flux with 405 nm emitted in the case of conductingrecording/reproducing of information for high density optical disk HD,red semiconductor laser LD2 (second light source) that emits the secondlight flux with 655 nm emitted in the case of conductingrecording/reproducing of information for DVD, photodetector PD12 that iscommon to the first light flux and the second light flux, beam formingelement BSH that forms a sectional shape of the laser light flux emittedfrom the violet semiconductor laser LD1 from an oval shape to a circularshape, objective optical system OBJ2 having a function to convergerespective light fluxes on information recording surface RL1 and oninformation recording surface RL2 respectively, biaxial actuator AC,first beam combiner BC1, second beam combiner BC2, first collimatingoptical system COL1, second collimating optical system COL2, diaphragmSTO, sensor lens SEN and liquid crystal element LCD.

When conducting recording/reproducing of information for high densityoptical disk HD in the optical pickup device PU2, violet semiconductorlaser LD1 is driven first to emit light as its light path is drawn withsolid lines in FIG. 4. A divergent light flux-emitted from the violetsemiconductor laser LD1 passes through the beam forming element BSH tobe formed therein from an oval shape to a circular shape in terms of itssectional form, then, it is transmitted through the first collimatingoptical system COLL to be converted into a collimated light flux. Afterthat, it passes successively through the first beam combiner BC1, thesecond beam combiner BC2 and liquid crystal element LCD, and isconverged by objective optical system OBJ2 on information recordingsurface RL1 through protective layer PL1 of high density optical disk HDto be a spot.

Incidentally, detailed explanation of objective optical system OBJ2 willbe given later.

The objective optical system OBJ2 conduct focusing and tracking throughoperations of biaxial actuator AC. The reflected light flux modulated oninformation recording surface RL1 by information pits passes againthrough the objective optical system OBJ2 and liquid crystal elementLCD, and is branched by the second beam combiner BC2, and is givenastigmatism when passing through sensor lens SEN, to be converged on alight-receiving surface of photodetector PD12. Thus, informationrecorded on high density optical disk HD by the use of output signals ofphotodetector PD12 can be read.

When conducting recording/reproducing of information for DVD, redsemiconductor laser LD2 is driven first to emit light as its light pathis drawn with dotted lines in FIG. 4. A divergent light flux emittedfrom the red semiconductor laser LD2 is transmitted through the firstcollimating optical system COL1 to be converted into a collimated lightflux, and then, is reflected on the first beam combiner BC1, and itpasses successively through the second beam combiner BC2 and liquidcrystal element LCD, to be converged by objective optical system OBJ2 oninformation recording surface RL2 through protective layer PL2 of DVD tobe a spot.

Then, the objective optical system OBJ2 conducts focusing and trackingthrough operations of biaxial actuator AC arranged on the peripherythereof. The reflected light flux modulated on information recordingsurface RL2 by information pits passes again through the objectiveoptical system OBJ2 and liquid crystal element LCD, then, is branched bythe second beam combiner BC2, to be given astigmatism when passingthrough sensor lens SEN, and it is converged on a light-receivingsurface of photodetector PD12. Thus, information recorded on DVD by theuse of output signals of photodetector PD2 can be read.

Next, the structure of the objective optical system OBJ2 will beexplained.

The objective optical system OBJ2 showing its side view in FIG. 5 is onewhose constituent element is an optical element of the invention. Theobjective optical system OBJ2 is composed of aberration correctingelement L1 representing a plastic lens and of light converging elementL2 representing an aspheric plastic lens with NA 0.85 having a functionto converge a laser light flux transmitted through the aberrationcorrecting element L1 on an information recording surface of an opticaldisk. Optical surface S1 of the aberration correcting element L1 closerto the laser light source is divided into first area AREA1 that includesan optical axis as shown in FIG. 6 (a) and second area AREA2 surroundinga circumference of the first area AREA1, and diffractive structure DOE(see an enlarged diagram in FIG. 5) composed of plural ring-shaped zonesis formed on the first area AREA1.

Optical surface S2 of the aberration correcting element L1 closer to anoptical disk is divided into third area AREA3 that includes an opticalaxis as shown in FIG. 6 (c) and fourth area AREA4 surrounding acircumference of the third area AREA3, and optical path differenceproviding structure PST composed of plural ring-shaped zones is formedon the third area AREA3. Incidentally, each of the first area AREA1 andthe third area AREA3 corresponds to an area within NA of DVD, and theaberration correcting element L1 and the light-converging element L2have respectively flange portion FL1 and flange portion FL2, each beingformed to be solid with the optical functional area, on peripheralportions of their optical functional areas (areas through which a lightflux emitted from a violet laser light source passes), and theaberration correcting element L1 and the light-converging element L2 areunited when the flange portions FL1 and FL2 are fitted each other.

The diffractive structure DOE is a structure for correcting sphericalaberration resulting from a difference of protective layer thicknessbetween a high density optical disk meeting a 0.1 mm standard and DVD,and the diffractive structure DOE is formed on an aspheric surface sothat the spherical aberration may be corrected properly. Further, a highdiffraction efficiency is secured on wavelength areas for both opticaldisks, by using 6^(th) order diffracted light as a beam forrecording/reproducing for a high density optical disk and by using4^(th) order diffracted light as a beam for recording/reproducing forDVD. Incidentally, when a laser light flux with a wavelength of about785 nm enters the diffractive structure DOE, 3^(rd) order diffractedlight is generated at a high diffraction efficiency of 80% or more, andtherefore, the objective optical system OBJ2 can be installed on anoptical pickup device having compatibility for high density optical diskHD, DVD and CD, in addition to an optical pickup device havingcompatibility for high density optical disk HD and DVD like the presentembodiment.

Since the diffractive structure DOE satisfies expressions (4) and (5),when a wavelength of a light flux entering the diffractive structure DOEis shifted toward the longer wavelength side by Δλ from designwavelength λ₁ on the high density optical disk side as shown withtwo-dot chain lines A1 in FIG. 7, the diffractive structure DOE hasspherical aberration characteristics wherein the spherical aberrationchanges toward insufficient correction direction.

Up to 70% within the first area AREA1, each ring-shaped zone of opticalpath difference providing structure PST is changed in a way that anoptical path length for the ring-shaped zone that is farther from theoptical axis is longer than that for the ring-shaped zone that is closerto the optical axis while, in the area outside the aforesaid range, eachring-shaped zone is changed in a way that an optical path length for thering-shaped zone that is farther from the optical axis is shorter thanthat for the ring-shaped zone that is closer to the optical axis, thus,the optical path difference providing structure PST has sphericalaberration characteristics that the spherical aberration changes towardthe excessive correction with shifting of incident light flux to thelonger wavelength side, as shown with dotted lines A2 in FIG. 7.

As stated above, with respect to the diffractive structure DOE thatgenerates beams having respectively different diffraction order numbersfor two wavelengths which are different greatly each other, an amount ofchanges in spherical aberration per a unit wavelength change is large.However, it is possible to cancel wavelength-dependency of sphericalaberration as shown with solid lines A3 in FIG. 7, by causing theoptical path difference providing structure PST to have sphericalaberration characteristics which are opposite to those of thediffractive structure DOE, so that wavelength-dependency of sphericalaberration of the diffractive structure DOE may be canceled.

Further, the diffractive structure DOE is blazed by prescribedwavelength λ_(B) within a range of λ₁-λ₂, and step Δ_(D) (μm) closest tothe optical axis of the diffractive structure DOE satisfies expression(6).

In addition, Δ_(D) (μm) closest to the optical axis of the optical pathdifference providing structure PST satisfies expressions (17) and (18),and is set to the depth which does not give the optical path differencesubstantially to design wavelength λ₁ on the high density optical diskside j design wavelength λ₂ on the DVD side and design wavelength λ₃ onthe CD side.

Since the diffractive structure DOE is formed only on the first areaAREA1, the second light flux passing through the second area AREA2 isgiven spherical aberration resulting from a difference of protectivelayer thickness between high density optical disk HD and DVD to be aflare component that does not contribute to formation of a spot oninformation recording surface RL2 of DVD. Since this is equivalent tothat objective optical system OBJ itself has an aperture restrictionfunction for DVD, optical pickup device PU2 does not need to be providedwith a diaphragm corresponding to NA₂ separately which makes itsstructure to be simple.

Incidentally, the objective optical system OBJ2 is united solidly withliquid crystal element LCD through connecting member C (see FIG. 4), andthe objective optical system OBJ2 conduct tracking and focusing togetherwith liquid crystal element LCD. In this case, diffractive indexdistribution is generated in a liquid crystal molecule layer whenvoltage is impressed on electrodes arranged to interpose the liquidcrystal molecule layer of liquid crystal element LCD, though anillustration thereof is omitted. On the wavefront that is transmittedthrough the liquid crystal molecule layer-on which the refractive indexdistribution is formed, there is added spherical aberration. In thiscase, when the refractive index distribution in the liquid crystalmolecule layer is controlled so that spherical aberration having a signopposite to that of spherical aberration changes of a spot oninformation recording surface RL1 of high density optical disk HD may beadded to the transmitted wavefront, the spot formed on informationrecording surface RL1 of high density optical disk HD can maintain thestate of corrected spherical aberration constantly, and excellentrecording/reproducing characteristics for high density optical disk HDcan be obtained.

The technology to correct spherical aberration changes by liquid crystalelement LCD of this kind is described, for example, in TOKKAI No.2001-43549, and detailed description is not given here.

Incidentally, causes for generation of spherical aberration to becorrected by liquid crystal element LCD include wavelength fluctuationscaused by errors in manufacturing of violet semiconductor lasers LD1,refractive index changes and refractive index distribution of objectiveoptical system OBJ resulting from temperature changes, focus jumpbetween layers in recording/reproducing for multi-layer disks such as2-layer disk and 4-layer disk and thickness fluctuations and thicknessdistribution caused by errors in manufacturing of protective layers PL1.

Further, spherical aberration of a spot formed on information recordingsurface RL2 of DVD may also be corrected by the liquid crystal elementLCD, which makes it possible to improve recording/reproducingcharacteristics for DVD.

By installing optical pickup device PU1 or PU2 shown respectively in thefirst or second embodiment, a rotation driving apparatus for holding anoptical disk rotatably and a control apparatus that controls driving ofthe respective apparatuses, it is possible to obtain an opticalinformation recording and reproducing apparatus capable of conducting atleast one of recording of optical information for an optical disk andreproducing of information recorded on an optical disk.

Example 1

Next, the example of an optical element that is optimum as the objectiveoptical system OBJ1 will be explained.

Table 1 shows lens data of an optical element, and FIG. 8 is a diagramof an optical path. The present example is composed of aberrationcorrecting element L1 on which diffractive structure DOE and opticalpath difference providing structure PST are formed and oflight-converging element L2. The aberration correcting element L1 is aplastic lens and the light-converging element L2 is a glass lens.

TABLE 1 (Optical specifications) HD: NA₁ = 0.85, f₁ = 1.76 mm, λ₁ = 405nm, m₁ = 0, t₁ = 0. 1 mm DVD: NA₂ = 0.65, f₂ = 1.823 mm, λ₂ = 655 nm, m₂= 0, t₂ = 0.6 mm CD: NA₃ = 0.45, f₃ = 1.824 mm, λ₃ = 785 nm, m₃ =−0.157, t₃ = 1.2 mm (Paraxial data) Surface r d₁ d₂ d₃ No. (mm) (mm)(mm) (mm) N_(λ1) N_(λ2) N_(λ3) ν_(d) OBJ ∞ ∞ 11.7000 STO 0.5000 0.50000.5000 1 −10.5405 1.0000 1.0000 1.0000 1.524694 1.506513 1.503235 56.5 2∞ 0.1000 0.1000 0.1000 3 1.2369 2.1400 2.1400 2.1400 1.622717 1.6031751.599244 61.2 4 −3.3104 0.5303 0.2794 0.1743 5 ∞ 0.1000 0.6000 1.20001.619495 1.577210 1.570423 30.0 6 ∞ (Aspheric surface coefficient) Firstsurface Third surface Fourth surface κ 0.00000E+00 −0.65471E+00−0.111004+03  A4 0.26599E−01  0.15588E−01 0.17200E+00 A6 0.25977E−02−0.10498E−02 −0.29168E+00  A8 0.28635E−02  0.10874E−01 0.37347E+00 A100.56637E−04 −0.10146E−01 −0.35736E+00  A12 0.00000E+00  0.30385E−020.19402E+00 A14 0.00000E+00  0.40266E−02 −0.43722E−01  A16 0.00000E+00−0.44036E−02 0.00000E+00 A18 0.00000E+00  0.17306E−02 0.00000E+00 A200.00000E+00 −0.25435E−03 0.00000E+00 (Diffraction order number,manufacture wavelength, optical path difference function coefficient)First surface n₁/n₂/n₃ 2/1/1 λB 390 nm B2 −0.12000E−01  B4 0.65766E−02B6 0.77632E−03 B8 0.66730E−03 B10 0.23534E−04 (optical path differenceproviding structure) i h_(iS) (mm) h_(iL) (mm) m_(i1d) (mm) m_(i1)m_(i2) m_(i3) 1 0.00000 0.40000 0.000000 0 0 0 2 0.40000 0.58000−0.015438 20 12 10 3 0.58000 0.72000 −0.030875 40 24 20 4 0.720000.86000 −0.046313 60 36 30 5 0.86000 1.29000 −0.061750 80 48 40 61.29000 1.36000 −0.046313 60 36 30 7 1.36000 1.40000 −0.030875 40 24 208 1.40000 1.44000 −0.015438 20 12 10 9 1.44000 1.47000 0.000000 0 0 0 101.47000 1.55000 0.015438 −20 −12 −10

The optical specifications in the case of using a high density opticaldisk include wavelength λ₁=405 nm, protective layer PL1 thickness t₁=0.1mm, numerical-aperture NA₁=0.85, focal length f₁=1.765 mm andmagnification m₁=0, while, the optical specifications in the case ofusing DVD include wavelength 2=655 nm, protective layer PL2 thicknesst₂=0.6 mm, numerical aperture NA₂=0.65, focal length f₂=1.823 mm andmagnification m₂=0, and the optical specifications in the case of usingCD include wavelength λ₃=785 nm, protective layer PL3 thickness t₃=1.2mm, numerical aperture NA₃=0.45, focal length f₃=1.824 mm andmagnification m₃=0.157.

An aspheric surface of the optical surface is expressed by the followingexpression Numeral 1 wherein a coefficient in Table 1 (Table 3 inExample 2) is substituted, when X (mm) represents an amount ofdeformation from a plane that is tangent to the aspheric surface at itsvertex, h (mm) represents a height in the direction perpendicular to theoptical axis and r (mm) represents a radius of curvature, wherein, κrepresents a conic constant and A_(2i) represents an aspheric surfacecoefficient.

$\begin{matrix}{x = {\frac{h^{2}/r}{1 + \sqrt{1 - {\left( {1 + k} \right)\left( {h/r} \right)^{2}}}} + {\sum\limits_{i = 2}{A_{2i}h^{2i}}}}} & \left( {{Numeral}\mspace{14mu} 1} \right)\end{matrix}$

In Table 1 (this applies also to lens data in the examples hereafter),NA₁, NA₂ and NA₃ represent numerical apertures respectively of highdensity optical disk HD, DVD and CD, f₁, f₂ and f₃ represent focallengths (mm) respectively of high density optical disk HD, DVD and CD,λ₁, λ₂ and λ₃ represent design wavelengths (nm) respectively of highdensity optical disk HD, DVD and CD, m₁, m₂ and m₃ representmagnifications respectively of high density optical disk HD, DVD and CD,t₁, t₂ and t₃ represent protective layer thicknesses respectively ofhigh density optical disk HD, DVD and CD, OBJ represents an object point(a luminous point of a semiconductor laser light source), STO representsa diaphragm, r represents a radius of curvature (mm), d₁, d₂ and d₃represent surface distances respectively of high density optical diskHD, DVD and CD, Nλ₁, Nλ₂ and Nλ₃ represent refractive indexes for designwavelengths respectively of high density optical disk HD, DVD and CD,ν_(d) represents Abbe's number for d line (587.6 nm), n₁, n₂ and n₃represent diffraction order numbers of beams for recording/reproducingrespectively of high density optical disk HD, DVD and CD, and λ_(B)represents a blaze wavelength (nm) of diffractive structure DOE.

A diffractive structure in each example is expressed by an optical pathdifference that is added to the transmitted wavelength by thisstructure. The optical path difference of this kind is expressed byoptical path difference function φ_(b) (mm) that is defined by thefollowing Numeral 2, when X represents a wavelength of an incident lightflux, h (mm) represent a height in the direction perpendicular to theoptical axis, B_(2j) represents an optical path difference functioncoefficient, n represents a diffraction order number and λ_(B)represents a manufacturing wavelength (which is also called a blazewavelength).

$\begin{matrix}{\Phi_{b} = {n \times {\lambda/\lambda_{B}} \times {\sum\limits_{j = 1}^{5}{B_{2j}h^{2j}}}}} & \left( {{Numeral}\mspace{14mu} 2} \right)\end{matrix}$

Further, diffractive structure DOE formed on optical surface S1 closerto the laser light source on aberration correcting element L1 is astructure that generates a second order diffracted light for wavelengthλ₁, a first order diffracted light for wavelength λ₂ and a first orderdiffracted light for wavelength λ₃. Diffraction efficienciesrespectively for λ₁, λ₂ and λ₃ are 97.7%, 93.4% and 99.1%, which meansthat the diffraction efficiency is high for every wavelength.

With respect to optical path difference providing structure PST formedon optical surface S2 closer to an optical disk on the aberrationcorrecting element L1, each step _(D)Δ_(P) is established to be in adepth satisfying Δ_(P)=m_(il)×λ₁×10−3/(N₁−1) (wherein, m_(il)=20) (inthe present example, Δ_(P)=0.015438 mm, because of N₁=1.524694). Opticalpath difference φ_(2P) to be added to λ₂ by each step Δ_(P) is asfollows,

$\quad\begin{matrix}{\Phi_{2P} = {\Delta_{P} \cdot {\left( {N_{2} - 1} \right)/\left( {\lambda_{2} \times 10^{- 3}} \right)}}} \\{= {20 \times \lambda_{1} \times {{10^{- 3}/\left( {N_{1} - 1} \right)} \cdot {\left( {N_{2} - 1} \right)/\left( {\lambda_{2} \times 10^{- 3}} \right)}}}} \\{= {20 \times \left( {\lambda_{1}/\lambda_{2}} \right) \times \left\lbrack {\left( {N_{2} - 1} \right)/\left( {N_{1} - 1} \right)} \right\rbrack}} \\{= {20 \times \left( {405/655} \right) \times \left\lbrack {\left( {1.506513 - 1} \right)/\left( {1.524694 - 1} \right)} \right\rbrack}} \\{= 11.93} \\{\approx 12}\end{matrix}$

and optical path difference φ_(3P) to be added to λ₃ by each step Δ_(P)is as follows.

$\quad\begin{matrix}{\Phi_{3P} = {\Delta_{P} \times {\left( {N_{3} - 1} \right)/\left( {\lambda_{3} \times 10^{- 3}} \right)}}} \\{= {\left\lbrack {20 \times \lambda_{1} \times {10^{- 3}/\left( {N_{1} - 1} \right)}} \right\rbrack \times \left\lbrack {\left( {N_{3} - 1} \right)/\left( {\lambda_{3} \times 10^{- 3}} \right)} \right\rbrack}} \\{= {20 \times \left( {\lambda_{1}/\lambda_{3}} \right) \times \left\lbrack {\left( {N_{3} - 1} \right)/\left( {N_{1} - 1} \right)} \right\rbrack}} \\{= {20 \times \left( {405/785} \right) \times \left\lbrack {\left( {1.503235 - 1} \right)/\left( {1.524694 - 1} \right)} \right\rbrack}} \\{= 9.90} \\{\approx 10}\end{matrix}$

Namely, each step Δ_(P) of the optical path difference providingstructure PST is established to be in a depth that does not give a phasedifference substantially to each of wavelengths λ₁, λ₂ and λ₃.

Incidentally, the symbol i in Table 1 shows the number of eachring-shaped zone of the optical path difference providing structure,wherein i=1 is for the ring-shaped zone including the optical axis, i=2is for the ring-shaped zone adjoining to the outside of the ring-shapedzone mentioned above (in the direction for getting away from the opticalaxis), and i=3 is for the ring-shaped zone adjoining to the outside ofthe ring-shaped zone mentioned immediately above, and the same ruleapplies correspondingly to the following. Namely, on the aberrationcorrecting element L1 in the present example, there are formed tenring-shaped zones. The symbols h_(is) and h_(iL) show respectively aheight of starting point and a height of ending point. The symbol mildrepresents an amount of deviation of each ring-shaped zone in theoptical axis direction, and its sign is “−” when the ring-shaped zone isshifted toward the laser light source side for the first ring-shapedzone (i=1), and its sign is “+” when the ring-shaped zone is shiftedtoward the optical disk side for the first ring-shaped zone (i=1). Forexample, the second ring-shaped zone (i=2) is shifted toward the laserlight source side for the first ring-shaped zone by an amount of0.015438 mm, and the tenth ring-shaped zone (i=10) is shifted toward theoptical disk side by an amount of 0.015438 mm for the first ring-shapedzone.

FIG. 9 is a graph showing wavefront aberration for wavelength λ₁+5 (nm)(=410 nm) under the assumption that the optical path differenceproviding structure PST is not formed in the present example, while,FIG. 10 is a graph showing wavefront aberration for wavelength λ₁+5 (nm)(=410 nm) in the present example. The diffractive structure DOE in thepresent example has spherical aberration characteristics whereinspherical aberration changes toward the excessive correction if awavelength of an incident light flux is shifted to the longer wavelengthside, and a third order spherical aberration component of the wavefrontaberration shown in FIG. 9 is 0.385 λRMS (λ=410 nm). In contrast tothis, a third order spherical aberration component of the wavefrontaberration shown in FIG. 10 is 0.030 λRMS (λ=410 nm), and it isunderstood that the spherical aberration characteristics of thediffractive structure DOE are canceled properly by the optical pathdifference providing structure PST having the spherical aberrationcharacteristics wherein spherical aberration changes toward theinsufficient correction if a wavelength of an incident light flux isshifted to the longer wavelength side.

Table 2 shows RMS values (square root of the sum of squares of sphericalaberration components for the ninth order or less) of wavefrontaberration for respective wavelengths λ₁ (nm), λ₁±5 (nm), λ₂ (nm), λ₂±5(nm) and λ₃ (nm), λ₃±5 (nm) in the present example. From this table, itis understood that the optical element of the present example has anexcellent performance for each of a high density optical disk, DVD andCD.

TABLE 2 High density optical disk application (NA₁ = 0.85) λ₁ (nm) 0.004λRMS λ₁ + 5 (nm) 0.033 λRMS λ₁ − 5 (nm) 0.033 λRMS DVD application (NA₂= 0.65) λ2 (nm) 0.005 λRMS λ₂ + 5 (nm) 0.025 λRMS λ₂ − 5 (nm) 0.020 λRMSCD application (NA₃ = 0.45) λ₃ (nm) 0.028 λRMS λ₃ + 5 (nm) 0.032 λRMS λ₃− 5 (nm) 0.025 λRMS

Example 2

Table 3 shows lens data of Example 2 of the invention, and FIG. 11 showsan optical path diagram. The present example is composed of the firstaberration correcting element L1 on which diffractive structure DOE isformed, the second aberration correcting element L2 on which opticalpath difference providing structure PST is formed and oflight-converging element L3. Each of the first aberration correctingelement L1 and the second aberration correcting element L2 is a plasticlens, and the light-converging element L3 is a glass lens.

TABLE 3 (Optical specifications) HD: NA₁ = 0.85, f₁ = 1.765 mm, λ₁ = 405nm, m₁ = 0, t₁ = 0.1 mm DVD: NA₂ = 0.65, f₂ = 1.807 mm, λ₂ = 655 nm, m₂= 0, t₂ = 0.6 mm (Paraxial data) Surface r d₁ d₂ No. (mm) (mm) (mm)N_(λ1) N_(λ2) ν_(d) OBJ ∞ ∞ STO 0.5000 0.5000 1 22.5785 1.0000 1.00001.524694 1.506513 56.5 2 ∞ 0.1000 0.1000 3 ∞ 1.0000 1.0000 1.5246941.506513 56.5 4 ∞ 0.1000 0.1000 5 1.2369 2.1400 2.1400 1.622717 1.60317561.2 6 −3.3104 0.5307 0.3015 7 ∞ 0.1000 0.6000 1.619495 1.577210 30.0 8∞ (Aspheric surface coefficient) First surface Fifth surface Sixthsurface κ 0.00000E+00 −0.65471E+00 −0.111004+03  A4 −0.62107E−02  0.15588E−01 0.17200E+00 A6 0.31256E−03 −0.10498E−02 −0.29168E+00  A8−0.15630E−02   0.10874E−01 0.37347E+00 A10 0.27716E−03 −0.10146E−01−0.35736E+00  A12 0.00000E+00  0.30385E−02 0.19402E+00 A14 0.00000E+00 0.40266E−02 −0.43722E−01  A16 0.00000E+00 −0.44036E−02 0.00000E+00 A180.00000E+00  0.17306E−02 0.00000E+00 A20 0.00000E+00 −0.25435E−030.00000E+00 (Diffraction order number, manufacture wavelength, opticalpath difference function coefficient) First surface n₁/n₂ 1/1 λB 490 nmB2 0.14000E−01 B4 −0.39555E−02  B6 0.21095E−03 B8 −0.99732E−03  B100.17650E−03 (Optical path difference providing structure) i h_(iS) (mm)h_(iL) (mm) m_(i1d) (mm) m_(i1) m_(i2) 1 0.00000 0.46000 0.000000 0 0 20.46000 0.66000 0.003859 −5 −3 3 0.66000 0.88000 0.007719 −10 −6 40.88000 1.30000 0.011578 −15 −9 5 1.30000 1.38000 0.007719 −10 −6 61.38000 1.44000 0.003859 −5 −3 7 1.44000 1.48000 0.000000 0 0 8 1.480001.55000 −0.003859 5 3

The optical specifications in the case of using a high density opticaldisk include wavelength λ₁=405 nm, protective layer PL1 thickness t₁=0.1mm, numerical aperture NA₁=0.85, focal length f₁=1.765 mm andmagnification m₁=0, while, the optical specifications in the case ofusing DVD include wavelength 2=655 nm, protective layer PL2 thicknesst₂=0.6 mm, numerical aperture NA₂=0.65, focal length f₂=1.807 mm andmagnification m₂=0.

Diffractive structure DOE formed on optical surface S1 of the firstaberration correcting element L1 is a structure that generates the firstorder diffracted light for both of wavelength λ₁ and λ₂, and its blazewavelength λ_(B) is 490 nm. Diffraction efficiencies for λ₁ and λ₂ arerespectively 83.5% and 79.0%, which means that a diffraction efficiencyis high for every wavelength.

With respect to optical path difference providing structure PST formedon optical surface S4 of the second aberration correcting element L2,each step Δ_(P) is established to be in a depth that satisfies

satisfying Δ_(P)=m_(il)×λ₁×10⁻³/(N₁−1) (wherein, m_(il)=5) (in thepresent example, Δ_(P)=0.003859 mm, because of N₁=1.524694). Opticalpath difference φ_(2P) to be added to λ₂ by each step Δ_(P) is asfollows.

$\quad\begin{matrix}{\Phi_{2P} = {\Delta_{P} \times {\left( {N_{2} - 1} \right)/\left( {\lambda_{2} \times 10^{- 3}} \right)}}} \\{= {\left\lbrack {5 \times \lambda_{1} \times {10^{- 3}/\left( {N_{1} - 1} \right)}} \right\rbrack \cdot \left\lbrack {\left( {N_{2} - 1} \right)/\left( {\lambda_{2} \times 10^{- 3}} \right)} \right\rbrack}} \\{= {5 \times \left( {\lambda_{1}/\lambda_{2}} \right) \times \left\lbrack {\left( {N_{2} - 1} \right)/\left( {N_{1} - 1} \right)} \right\rbrack}} \\{= {5 \times \left( {405/655} \right) \times \left\lbrack {\left( {1.506513 - 1} \right)/\left( {1.524694 - 1} \right)} \right\rbrack}} \\{= 2.98} \\{\approx 3}\end{matrix}$

Namely, each step Δ_(P) of the optical path difference providingstructure PST is in a depth that does not give a phase differencesubstantially to both of λ₁ and λ₂ Incidentally, the symbol i in Table 3shows the number of each ring-shaped zone of the optical path differenceproviding structure, wherein i=1 is for the ring-shaped zone includingthe optical axis, i=2 is for the ring-shaped zone adjoining to theoutside of the ring-shaped zone mentioned above (in the direction forgetting away from the optical axis), and i=3 is for the ring-shaped zoneadjoining to the outside of the ring-shaped zone mentioned immediatelyabove, and the same rule applies correspondingly to the following.Namely, on the aberration correcting element L1 in the present example,there are formed eight ring-shaped zones. The symbols h_(is) and h_(iL)show respectively a height of starting point and a height of endingpoint. The symbol m_(ild) represents an amount of deviation of eachring-shaped zone in the optical axis direction, and its sign is “−” whenthe ring-shaped zone is shifted toward the laser light source side forthe first ring-shaped zone (i=1), and its sign is “+” when thering-shaped zone is shifted toward the optical disk side for the firstring-shaped zone (i=1). For example, the second ring-shaped zone (i=2)is shifted toward the laser light source side for the first ring-shapedzone by an amount of 0.003859 mm, and the eighth ring-shaped zone (i=8)is shifted toward the optical disk side by an amount of 0.003859 mm forthe first ring-shaped zone.

FIG. 12 is a graph showing wavefront aberration for wavelength λ₁+5 (nm)(=410 nm) under the assumption that the optical path differenceproviding structure PST is not formed in the present example, while,FIG. 13 is a graph showing wavefront aberration for wavelength λ₁+5 (nm)(=410 nm) in the present example. The diffractive structure DOE in thepresent example has spherical aberration characteristics whereinspherical aberration changes toward the insufficient correction if awavelength of an incident light flux is shifted to the longer wavelengthside, and a third order spherical aberration component of the wavefrontaberration shown in FIG. 12 is 0.063 λRMS (λ=410 nm). In contrast tothis, a third order spherical aberration component of the wavefrontaberration shown in FIG. 13 is 0.013 λRMS (λ=410 nm), and it isunderstood that the spherical aberration characteristics of thediffractive structure DOE are canceled properly by the optical pathdifference providing structure PST having the spherical aberrationcharacteristics wherein spherical aberration changes toward theexcessive correction if a wavelength of an incident light flux isshifted to the longer wavelength side.

Table 4 shows RMS values (square root of the sum of squares of sphericalaberration components for the ninth order or less) of wavefrontaberration for respective wavelengths λ₁ (nm), λ₁±5 (nm), λ₂ (nm) andλ₂±5 (nm) in the present example. From this table, it is understood thatthe optical element of the present example has an excellent performancefor each of a high density optical disk and DVD.

TABLE 4 High density optical disk application (NA₁ = 0.85) λ₁ (nm) 0.000λRMS λ₁ + 5 (nm) 0.015 λRMS λ₁ − 5 (nm) 0.019 λRMS DVD application (NA₂= 0.65) λ₂ (nm) 0.001 λRMS λ₂ + 5 (nm) 0.007 λRMS λ₂ − 5 (nm) 0.007 λRMS

Example 3

Table 5 shows lens data of Example 3 of the invention, and FIG. 14 showsan optical path diagram. The present example is an objective opticalsystem wherein a light-converging element that is made up of one grouphas a function of the optical element of the invention, and it is aplastic lens wherein diffractive structure DOE is formed on opticalsurface S1 of the plastic lens closer to the laser light source, andoptical path difference providing structure PST is formed on opticalsurface S2 of the plastic lens closer to an optical disk.

TABLE 5 (Optical specifications) HD: NA₁ = 0.67, f₁ = 2.986 mm, λ₁ = 405nm, m₁ = 0, t₁ = 0.6 mm DVD: NA₂ = 0.655, f₂ = 3.070 mm, λ₂ = 655 nm, m₂= 0, t₂ = 0.6 mm (Paraxial data) Surface r d₁ d₂ No. (mm) (mm) (mm)N_(λ1) N_(λ1) ν_(d) OBJ ∞ ∞ STO 0.5000 0.5000 1 2.0249 2.3000 2.30001.567015 1.547023 55.0 2 −9.1652 1.3020 1.3590 3 ∞ 0.6000 0.60001.619495 1.577210 30.0 4 ∞ (Aspheric surface coefficient) Second surfaceFirst ring- Second ring- First surface shaped zone shaped zone κ−0.75342E+00  −0.16826E+03  −0.36374E+02  A4 0.33234E−02 0.22632E−020.17512E−01 A6 0.59252E−03 −0.47228E−02  −0.47228E−02  A8 −0.10732E−03 0.54260E−03 0.54260E−03 A10 0.41199E−04 −0.21717E−04  −0.21717E−04  A12−0.60853E−05  0.00000E+00 0.00000E+00 A14 0.00000E+00 0.00000E+000.00000E+00 A16 0.00000E+00 0.00000E+00 0.00000E+00 A18 0.00000E+000.00000E+00 0.00000E+00 A20 0.00000E+00 0.00000E+00 0.00000E+00 Secondsurface Third ring− Fourth ring− shaped zone shaped zone κ −0.41644E+02 −0.45860E+02  A4 0.17208E−01 0.17052E−01 A6 −0.47228E−02  −0.47228E−02 A8 0.54260E−03 0.54260E−03 A10 −0.21717E−04  −0.21717E−04  A120.00000E+00 0.00000E+00 A14 0.00000E+00 0.00000E+00 A16 0.00000E+000.00000E+00 A18 0.00000E+00 0.00000E+00 A20 0.00000E+00 0.00000E+00(Diffraction order number, manufacture wavelength, optical pathdifference function coefficient) First surface n₁/n₂ 3/2 λB 420 nm B2−0.35000E−02 B4 −0.32626E−03 B6 −0.23517E−04 B8  0.54729E−06 B10−0.72504E−06 (Optical path difference providing structure) i h_(iS) (mm)h_(iL) (mm) m_(i1d) (mm) m_(i1) m_(i2) 1 0.00000 0.50000 0.000000 0 0 20.50000 1.20000 0.003554 −5 −3 3 1.20000 1.47000 0.000000 0 0 4 1.470001.60000 −0.003706 5 3

The optical specifications in the case of using a high density opticaldisk include wavelength λ₁=405 nm, protective layer PL1 thickness t₁=0.6mm, numerical aperture NA₁=0.67, focal length f₁=2.986 mm andmagnification m₁=0, while, the optical specifications in the case ofusing DVD include wavelength λ₂=655 nm, protective layer PL2 thicknesst₂=0.6 mm, numerical aperture NA₂=0.656, focal length f₂=3.070 mm andmagnification m₂=0.

The diffractive structure DOE is of the structure that generates a thirdorder diffracted light for wavelength λ₁ and generates a second orderdiffracted light for wavelength λ₂, and its blaze wavelength λB is 420nm. Diffraction efficiencies for λ₁ and λ₂ are respectively 95.0% and94.0%, which means that a diffraction efficiency is high for everywavelength.

With respect to the optical path difference providing structure PST,each step Δ_(P) is established to be in a depth that satisfiesΔ_(P)=m_(il)×λ1×10⁻³/(N₁−1) (wherein, m_(il)=5) (in the present example,Δ_(P)=0.003571 mm, because of N₁=1.567015). Optical path differenceφ_(2P) to be added to λ₂ by each step Δ_(P) is as follows.

$\quad\begin{matrix}{\Phi_{2P} = {\Delta_{P} \times {\left( {N_{2} - 1} \right)/\left( {\lambda_{2} \times 10^{- 3}} \right)}}} \\{= {\left\lbrack {5 \times \lambda_{1} \times {10^{- 3}/\left( {N_{1} - 1} \right)}} \right\rbrack \cdot \left\lbrack {\left( {N_{2} - 1} \right)/\left( {\lambda_{2} \times 10^{- 3}} \right)} \right\rbrack}} \\{= {5 \times \left( {\lambda_{1}/\lambda_{2}} \right) \times \left\lbrack {\left( {N_{2} - 1} \right)/\left( {N_{1} - 1} \right)} \right\rbrack}} \\{= {5 \times \left( {405/655} \right) \times \left\lbrack {\left( {1.547023 - 1} \right)/\left( {1.567015 - 1} \right)} \right\rbrack}} \\{= 2.98} \\{\approx 3}\end{matrix}$

Namely, each step Δ_(P) of the optical path difference providingstructure PST is in a depth that does not give a phase differencesubstantially to both of λ₁ and λ₂. However, in the present example, theoptical path difference providing structure PST is formed on an opticalsurface which has a refractive power and which convergence light fluxenters into. Therefore, steps of every ring-shaped zones and asphericshapes of every ring-shaped zones are designed so that a sphericalaberration for a wavelength λ₁ may takes the minimum in consideration ofan incident angle variation of the incident light flux and a passingdirection variation of a refracted light flux. Accordingly, mild, adepth each ring-shaped zone of the optical path difference providingstructure PST shown in Table 5 slightly differs from the valuecalculated from above-mentioned Δ_(P)=0.003571 mm.

Incidentally, the symbol i in Table 5 shows the number of eachring-shaped zone of the optical path difference providing structure,wherein i=1 is for the ring-shaped zone including the optical axis, i=2is for the ring-shaped zone adjoining to the outside of the ring-shapedzone mentioned above (in the direction for getting away from the opticalaxis), and i=3 is for the ring-shaped zone adjoining to the outside ofthe ring-shaped zone mentioned immediately above, and the same ruleapplies correspondingly to the following. Namely, oh the objectiveoptical element in the present example, there are formed fourring-shaped zones. The symbols h_(is) and h_(iL) show respectively aheight of starting point and a height of ending point. The symbol mildrepresents an amount of deviation of each ring-shaped zone in theoptical axis direction, and its sign is “−” when the ring-shaped zone isshifted toward the laser light source side for the first ring-shapedzone (i=1), and its sign is “+” when the ring-shaped zone is shiftedtoward the optical disk side for the first ring-shaped zone (i=1). Forexample, the second ring-shaped zone (i=2) is shifted toward the laserlight source side for the first ring-shaped zone by an amount of0.003571 mm, and the fourth ring-shaped zone (i=4) is shifted toward theoptical disk side by an amount of 0.003571 mm for the first ring-shapedzone.

Besides, in the present example, the displacement of each ring-shapedzone along to the optical axis m_(il)d shows the displacement of anintersection of an extended line of the i-th aspherical surface and theoptical axis from an intersection of an extended line of the firstaspherical surface and the optical axis, as shown in FIG. 15.

FIG. 16 is a graph showing wavefront aberration for wavelength λ₁+5 (nm)(=410 nm) under the assumption that the optical path differenceproviding structure PST is not formed in the present example, while,FIG. 17 is a graph showing wavefront aberration for wavelength λ₁+5 (nm)(=410 nm) in the present example. The diffractive structure DOE in thepresent example has spherical aberration characteristics whereinspherical aberration changes toward the insufficient correction if awavelength of an incident light flux is shifted to the longer wavelengthside, and a third order spherical aberration component of the wavefrontaberration shown in FIG. 16 is 0.025 λRMS (λ=410 nm). In contrast tothis, a third order spherical aberration component of the wavefrontaberration shown in FIG. 17 is 0.005 λRMS (λ=410 nm), and it isunderstood that the spherical aberration characteristics of thediffractive structure DOE are canceled properly by the optical pathdifference providing structure PST having the spherical aberrationcharacteristics wherein spherical aberration changes toward theexcessive correction if a wavelength of an incident light flux isshifted to the longer wavelength side.

Table 6 shows RMS values (square root of the sum of squares of sphericalaberration components for the ninth order or less) of wavefrontaberration for respective wavelengths λ₁ (nm), λ₁±5 (nm), λ₂ (nm) andλ₂±5 (nm) in the present example. From this table, it is understood thatthe optical element of the present example has an excellent performancefor each of a high density optical disk and DVD.

TABLE 6 High density optical disk application (NA₁ = 0.67) λ₁ (nm) 0.003λRMS λ₁ + 5 (nm) 0.006 λRMS λ₁ − 5 (nm) 0.012 λRMS DVD application (NA₂= 0.655) λ₂ (nm) 0.009 λRMS λ₂ + 5 (nm) 0.007 λRMS λ₂ − 5 (nm) 0.012λRMS

1. An optical element for use in an optical pickup device to conductreproducing and/or recording information for a first disk including aprotective substrate having a thickness t₁ by the use of a first lightflux having a wavelength λ₁(nm) emitted from a first light source and toconduct reproducing and/or recording information for a second diskincluding a protective substrate having a thickness t₂ (t₂≧t₁) by theuse of a second light flux having a wavelength λ₂(λ₂>λ₁) (nm) emittedfrom a second light source, the optical element comprising: an opticalsurface on which a first phase structure is formed to have a function tocorrect a spherical aberration caused by a difference in thickness ofthe protective substrate between the first optical disk and the secondoptical disk or a function to correct a spherical aberration caused by adifference in wavelength between the first light flux and the secondlight flux; an optical surface on which a second phase structure isformed such that when the wavelength of the first light flux changes,the second phase structure generates a spherical aberration in adirection reverse to a direction of a spherical aberration generated bythe first phase structure, wherein the first phase structure is adiffractive structure that generates an n₁ ^(th) order diffracted ray asa diffracted ray having a maximum diffraction efficiency when the firstlight flux comes in and generates an n₂ ^(th) order diffracted ray(|n₁≧|n₂|) as a diffracted ray having the maximum diffraction efficiencywhen the second light flux comes in, and wherein when N₁ and N₂ arerefractive indexes of the optical element for the first light fluxhaving the wavelength λ₁ and the second light flux having the wavelengthλ₂ respectively and INT (X) is an integer closest to X, the followingformulas are satisfied:INT(δφ_(D))−δφ_(D)>0δφ_(D) ={n ₁×λ₁/(N ₁−1)}/{n ₂×λ₂ /N ₂−1)}, and wherein the first phasestructure has a spherical aberration characteristic such that when awavelength of an incident light flux shifts to a longer wavelength side,a spherical aberration changes to be under corrected.
 2. The opticalelement of claim 1, wherein the second phase structure is an opticalpath difference providing structure including a plurality of ring-shapedzones divided with stepped sections each formed in an optical axisdirection.
 3. The optical element of claim 1, wherein when thewavelength of the first light flux changes within a range of (λ₁−5) (nm)to (λ₁+5) (nm), the second phase structure has a function to generate aspherical aberration in a direction reverse to the direction of aspherical aberration generated by the first phase structure.
 4. Theoptical element of claim 1, wherein the second phase structure has aspherical aberration characteristic such that when the wavelength of thefirst light flux shifts to a longer wavelength side within a range of(λ₁−5) (nm) to (λ₁+5) (nm), a spherical aberration changes to be overcorrected.
 5. The optical element of claim 1, wherein the first phasestructure is formed on an aspherical surface formed such that as aposition of an optical path on the aspherical surface is distant morefrom an optical axis, a length of the optical path becomes longer. 6.The optical element of claim 1, wherein when an added amount of anoptical path length by the first phase structure is defined by thefollowing formula with optical path difference function coefficients B₂,B₄, B₆, B₈, B₁₀ . . . , and a diffraction order n:φ_(b) =n×(B ₂ h ² +B ₄ h ⁴ +B ₆ h ⁶ +B ₈ h ⁸ +B ₁₀ h ¹⁰+ . . . ) B₂ andB₄ have a different sign from each other and h is a height.
 7. Theoptical element of claim 1, wherein on the optical surface on which thefirst phase structure is formed, the first phase structure is formedwithin at least a range of 0% to 70% of a maximum effective diameter ofthe optical surface and the first phase structure is not formed withinat least a range of 85% to 100% of the maximum effective diameter. 8.The optical element of claim 1, wherein on the optical surface on whichthe second phase structure is formed, the second phase structure isformed within at least a range of 0% to 70% of a maximum effectivediameter of the optical surface and the second phase structure is notformed within at least a range of 85% to 100% of the maximum effectivediameter.
 9. The optical element of claim 4, wherein the second phasestructure does not provide an optical path difference for the firstlight flux having the wavelength λ₁ and provides an optical pathdifference for the first light flux having a wavelength (λ₁+5) (nm) andthe first light flux having a wavelength (λ₁−5) (nm).
 10. The opticalelement of claim 1, wherein the following formulas are satisfied:|λ₂−λ₁|>50 nm and |n ₁ |>|n ₂|.
 11. The optical element of claim 10,wherein λ₁ is within a range of 350 nm to 450 nm, λ₂ is within a rangeof 600 nm to 700 nm, and a combination (n₁, n₂) of n₁ and n₂ satisfiesthe following formula:(n₁,n₂)=(2,1),(3,2),(5,3),(8,5) or (10,6).
 12. The optical element ofclaim 11, wherein the optical element is made of a material whoserefractive index for the first light flux having the wavelength λ₁ iswithin a range of 1.5 to 1.6 and Abbe constant for d-line (587.6 nm) iswithin a range of 50 to 60 and the diffractive structure includes apredetermined number of ring-shaped zones divided with stepped sectionseach formed in an optical axis direction, and a stepped section Δ_(D)(μm) closest to an optical axis among the stepped sections satisfies oneof the following formulas:1.25<Δ_(D)<1.652.05<Δ_(D)<2.553.40<Δ_(D)<4.105.70<Δ_(D)<6.457.00<Δ_(D)<8.00.
 13. The optical element of claim 10, wherein the secondphase structure is an optical path difference providing structureincluding a plurality of ring-shaped zones divided with stepped sectionseach formed in an optical axis direction, and when Δ_(P) (μm) is astepped section closest to an optical axis among the stepped sections inthe second phase structure, N₁ and N₂ are refractive indexes of theoptical element for the first light flux having the wavelength λ₁ andthe second light flux having the wavelength λ₂ respectively and INT(X)is an integer closest to X, the following formulas are satisfied:0≦|INT(φ_(1P))−φ_(1P)|≦0.40≦|INT(φ_(2P))−φ_(2P)|≦0.4φ_(1P)=Δ_(P)·(N ₁−1)/(λ₁×10⁻³)φ_(2P)=Δ_(P)·(N ₂−1)/(λ₂×10⁻³)
 14. The optical element of claim 13,wherein when λ₁ is within a range of 350 nm to 450 nm, λ₂ is within arange of 600 nm to 700 nm, the optical element is made of a materialwhose refractive index for the first light flux having the wavelength λ₁is within a range of 1.5 to 1.6 and Abbe constant for d-line (587.6 nm)is within a range of 50 to 60 and p is a positive integer, the followingformulas are satisfied:INT(φ_(1P))=5pINT(φ_(2P))=3p.
 15. The optical element of claim 1, wherein the firstphase structure generates an n₃ ^(th) order diffracted ray (|n₂|≧|n₃|)when a third light flux having a wavelength λ₃ (λ₃>λ₂) (nm) comes in.16. The optical element of claim 15, wherein λ₁ is within a range of 350nm to 450 nm, λ₂ is within a range of 600 nm to 700 nm, λ₃ is within arange of 700 nm to 850 nm, and a combination (n₁, n₂, n₃) of n₁, n₂ andn₃ satisfies the following formula:(n₁,n₂,n₃)=(2,1,1),(8,5,4), or (10,6,5).
 17. The optical element ofclaim 16, wherein the optical element is made of a material whoserefractive index for the first light flux having the wavelength λ₁ iswithin a range of 1.5 to 1.6 and Abbe constant for d-line (587.6 nm) iswithin a range of 50 to 60 and the diffractive structure includes apredetermined number of ring-shaped zones divided with stepped sectionseach formed in an optical axis direction, and a stepped section Δ_(D)(μm) closest to an optical axis among the stepped sections satisfies oneof the following formulas:1.25<Δ_(D)<1.655.70<Δ_(D)<6.457.00<Δ_(D)<8.00.
 18. The optical element of claim 15, wherein the secondphase structure is an optical path difference providing structureincluding a plurality of ring-shaped zones divided with stepped sectionseach formed in an optical axis direction, and when Δ_(P) (μm) is astepped section closest to an optical axis among the stepped sections inthe second phase structure, N₁, N₂ and N₃ are the refractive indexes ofthe optical element for the first light flux having the wavelength λ₁,the second light flux having the wavelength λ₂ and the third light fluxhaving the wavelength λ₃ respectively and INT(X) is an integer closestto X, the following formulas are satisfied:0≦|INT(φ_(1P))−φ_(1P)≦0.40≦|INT(φ_(2P))−φ_(2P)≦0.40≦|INT(φ_(3P))−φ_(3P)≦0.4φ_(1P)=Δ_(P)×(N ₂−1)/(λ₁×10⁻³)φ_(2P)=Δ_(P)×(N ₂−1)/(λ₂×10⁻³)φ_(3P)=Δ_(P)×(N ₃−1)/(λ₃×10⁻³)
 19. The optical element of claim 18,wherein when λ₁ is within a range of 350 nm to 450 nm, λ₂ is within arange of 600 nm to 700 nm, λ₃ is within a range of 700 nm to 850 nm, theoptical element is made of a material whose refractive index for thefirst light flux having the wavelength λ₁ is within a range of 1.5 to1.6 and Abbe constant for d-line (587.6 nm) is within a range of 50 to60 and p is a positive integer, the following formulas are satisfied:INT(φ_(1P))=10pINT(φ_(2P))=6pINT(+_(3P))=5p.
 20. The optical element of claim 1, wherein the opticalelement comprises a first structural element on which the first phasestructure is formed and a second structural element on which the secondphase structure is formed.
 21. The optical element of claim 1, whereinthe optical element is a plastic lens.
 22. The optical element of claim1, wherein the optical element is a structural element of an objectiveoptical system for use in an optical pickup device.
 23. The opticalelement of claim 22, wherein the objective optical system comprises anaberration correcting element and a light converging element to convergea light flux emitted from the aberration correcting element on aninformation recording surface of an optical disk.
 24. The opticalelement of claim 22, wherein the objective optical system is onegroup-structured light converging element and the light convergingelement is the optical element.
 25. An optical pickup device to conductreproducing and/or recording information for a first disk including aprotective substrate having a thickness t₁ by the use of a first lightflux having a wavelength λ₁ (nm) emitted from a first light source andto conduct reproducing and/or recording information for a second diskincluding a protective substrate having a thickness t₂ (t₂≧t₁) by theuse of a second light flux having a wavelength λ₂ (λ₂>λ₁) (nm) emittedfrom a second light source, the optical pickup device comprises theoptical element described in claim
 1. 26. The optical pickup device ofclaim 25, wherein the optical pickup device further conducts reproducingand/or recording information for a third disk including a protectivesubstrate having a thickness t₃ (t₃≧t₂) by the use of a third light fluxhaving a wavelength λ₃ (λ₃>λ₂) (nm) emitted from a third light source,and the third light flux comes in the optical element on a state of adivergent light flux.
 27. An optical information recording reproducingapparatus, comprising: the optical pickup device described in claim 25so as to conduct at least one of recording information for an opticaldisk and reproducing information for the optical disk.
 28. An opticalelement for use in an optical pickup device to conduct reproducingand/or recording information for a first disk including a protectivesubstrate having a thickness t₁ by the use of a first light flux havinga wavelength λ₁ (nm) emitted from a first light source and to conductreproducing and/or recording information for a second disk including aprotective substrate having a thickness t₂ (t₂≧t₁) by the use of asecond light flux having a wavelength λ₂ (λ₂>λ₁) (nm) emitted from asecond light source, the optical element comprising: an optical surfaceon which a diffractive structure is formed to generate an n₁ ^(th) orderdiffracted ray as a diffracted ray having an maximum diffractionefficiency when the first light flux comes in and to generate an n₂^(th) order diffracted ray (|n₁|≧|n₂|) as a diffracted ray having themaximum diffraction efficiency when the second light flux comes in; andan optical surface on which an optical path difference providingstructure including a plurality of ring-shaped zones divided withstepped sections each formed in an optical axis direction is formed,wherein when the wavelength of the first light flux changes within arange of (λ₁−5) (nm) to (λ₁+5) (nm), the optical path differenceproviding structure has a function to generate a spherical aberration ina direction reverse to a direction of a spherical aberration generatedby the diffractive structure, wherein when N₁ and N₂ are refractiveindexes of the optical element for the first light flux having thewavelength λ₁ and the second light flux having the wavelength λ₂respectively and INT(X) is an integer closest to X, the followingformulas are satisfied:INT(δφ_(D))−δφ_(D)>0δφ_(D) ={n ₁×λ₁/(N ₁−1)}/{n ₂×λ₂/(N ₂−1)}, and wherein the diffractivestructure has a spherical aberration characteristic such that when awavelength of an incident light flux shifts to a longer wavelength side,a spherical aberration changes to be under corrected.
 29. The opticalelement of claim 28, wherein the diffractive structure includes aplurality of ring-shaped zones divided with stepped sections each formedin the optical axis direction and when Δ_(D) (μm) is a stepped sectionclosest to an optical axis among the stepped sections, a manufacturingwavelength is λ_(B) (nm) (λ₁<λ_(B)<λ₂), N_(B) is a refractive index ofthe optical element for the manufacturing wavelength λ_(B), thefollowing formula is satisfied:Δ_(D) =n ₁·λ_(B)×10⁻³/(N _(B)−1).
 30. The optical element of claim 28,wherein the optical path difference providing structure has a sphericalaberration characteristic such that when the wavelength of the firstlight flux shifts to a longer wavelength side within a range of (λ₁-5)(nm) to (λ₁+5) (nm), a spherical aberration changes to be overcorrected.
 31. The optical element of claim 28, wherein the optical pathdifference providing structure is formed on an aspherical surface formedsuch that as a position of an optical path on the aspherical surface isdistant more from an optical axis, the length of the optical pathbecomes longer.
 32. The optical element of claim 28, wherein on theoptical surface on which the optical path difference providing structureis formed, the diffractive structure is formed within at least a rangeof 0% to 70% of a maximum effective diameter of the optical surface andthe diffractive structure is not formed within at least a range of 85%to 100% of the maximum effective diameter.
 33. The optical element ofclaim 28, wherein the optical element is made of a material whoserefractive index for the first light flux having the wavelength λ₁ iswithin a range of 1.5 to 1.6 and Abbe constant for d-line (587.6 nm) iswithin a range of 50 to 60 and the diffractive structure includes apredetermined number of ring-shaped zones divided with stepped sectionseach formed in the optical axis direction, and a stepped section Δ_(D)(μm) closest to an optical axis among the stepped sections satisfies oneof the following formulas:1.25<Δ_(D)<1.652.05<Δ_(D)<2.553.04<Δ_(D)<4.105.70<Δ_(D)<6.457.00<Δ_(D)<8.00.
 34. The optical element of claim 28, wherein when λ₁ iswithin a range of 350 nm to 450 nm, λ₂ is within a range of 600 nm to700 nm, the optical element is made of a material whose refractive indexfor the first light flux having the wavelength λ₁ is within a range of1.5 to 1.6 and Abbe constant for d-line (587.6 nm) is within a range of50 to 60, Δ_(P) (μm) is a stepped section closest to an optical axisamong the stepped sections in the optical path difference providingstructure, N₁ and N₂ are refractive indexes of the optical element forthe first light flux having the wavelength λ₁ and the second light fluxhaving the wavelength λ₂ respectively and INT(X) is an integer closestto X and p is a positive integer, the following formulas are satisfied:φ_(1P)=Δ_(P) x(N ₁−1)/(λ₁×10⁻³)φ_(2P)=Δ_(P) ·x(N ₂−1)/(λ₂×10⁻³)INT(φ_(1P))=5pINT(φ_(2P))=3p.
 35. The optical element of claim 28, wherein thediffractive structure generates an n₃ ^(th) order diffracted ray(|n₂|≧|n₃|) when a third light flux having a wavelength λ₃ (λ₃>λ₂) comesin.
 36. The optical element of claim 35, wherein λ₁ is within a range of350 nm to 450 nm, λ₂ is within a range of 600 nm to 700 nm, λ₃ is withina range of 700 nm to 850 nm, and a combination (n₁, n₂, n₃) of n₁, n₂and n₃ satisfies the following formula:(n₁,n₂,n₃)=(2,1,1),(8,5,4), or (10,6,5).
 37. The optical element ofclaim 36, wherein the optical element is made of a material whoserefractive index for the first light flux having the wavelength λ₁ iswithin a range of 1.5 to 1.6 and Abbe constant for d-line (587.6 nm) iswithin a range of 50 to 60 and the diffractive structure includes apredetermined number of ring-shaped zones divided with stepped sectionseach formed in the optical axis direction, and a stepped section Δ_(D)(μm) closest to an optical axis among the stepped sections satisfies oneof the following formulas:1.25<Δ_(D)<1.655.70<Δ_(D)<6.457.00<Δ_(D)<8.00.
 38. The optical element of claim 35, wherein when Δ_(P)(μm) is a stepped section closest to an optical axis among the steppedsections in the optical path difference providing structure, N₁, N₂ andN₃ are refractive indexes of the optical element for the first lightflux having the wavelength λ₁, the second light flux having thewavelength λ₂ and the third light flux having the wavelength λ₃respectively and INT(X) is an integer closest to X, the followingformulas are satisfied:0≦|INT(φ_(1P))−φ_(1P)|≦0.40≦|INT(φ_(2P))−φ_(2P)|≦0.40≦|INT(φ_(3P))−φ_(3P)|≦0.4φ_(1P)=Δ_(P)×(N ₁−1)/(λ₁×10⁻³)φ_(2P)=Δ_(P)×(N ₂−1)/(λ₂×10⁻³)φ_(3P)=Δ_(P)×(N ₃−1)/(λ₃×10⁻³)
 39. The optical element of claim 38,wherein when λ₁ is within a range of 350 nm to 450 nm, λ₂ is within arange of 600 nm to 700 nm, λ₃ is within a range of 700 nm to 850 nm, theoptical element is made of a material whose refractive index for thefirst light flux having the wavelength λ₁ is within a range of 1.5 to1.6 and Abbe constant for d-line (587.6 nm) is within a range of 50 to60 and p is a positive integer, the following formulas are satisfied:INT(φ_(1P))=10pINT(φ_(2P))=6pINT(φ_(3P))=5p.
 40. The optical element of claim 28, wherein the opticalelement is a plastic lens.
 41. An optical pickup device to conductreproducing and/or recording information for a first disk including aprotective substrate having a thickness t₁ by the use of a first lightflux having a wavelength λ₁ (nm) emitted from a first light source, toconduct reproducing and/or recording information for a second diskincluding a protective substrate having a thickness t₂ (t₂≧t₁) by theuse of a second light flux having a wavelength λ₂ (λ₂>λ₁) (nm) emittedfrom a second light source, and to conduct reproducing and/or recordinginformation for a third disk including a protective substrate having athickness t₃ (t₃≧t₂) by the use of a third light flux having awavelength λ₃ (λ₃>λ₂) (nm) emitted from a third light source, theoptical pickup device comprises the optical element described in claim28 and the third light flux comes in the optical element on a state of adivergent light flux.